This is a happy ending not a sad ending. I don't know why some people keep being convinced that this is a dream or an after life vision.

Xiao Heng survived because he killed their ruler

In The Art Of War morals laws are important. Obeying  the ruler to death  is a must.
On the battlefields, the person that counts is the ruler. Once the ruler is dead, the army surrenders or retreats.

See The art of war University of toledo :

-  The battle is won by the ruler . When the ruler is dead the army retreats:

"Terrain :
11. If the enemy has occupied them before you, do not follow him, but retreat and try to entice him away.
[The turning point of Li Shih-min's campaign in 621 A.D. against the two rebels, Tou Chien-te, King of Hsia,and Wang Shih-ch`ung, Prince of Cheng, was his seizure of the heights of Wu-lao, in spike of which TouChien-te persisted in his attempt to relieve his ally in Lo-yang, was defeated and taken prisoner. See CHIUT`ANG, ch. 2, fol. 5 verso, and also ch. 54.

- Morals laws are important. A soldier must obey the leader. This is why if the leader looses  as shown above with the prince and the king. The soldiers surrender or retreat ( regardless of their number).

" 3. The art of war, then, is governed by five constant factors, to be taken into account in one's deliberations,when seeking to determine the conditions obtaining in the field.
4. These are:
(1) The Moral Law; (2) Heaven; (3) Earth; (4) The Commander; (5) Method and discipline.

The MORAL LAW causes the people to be in complete accord with their ruler, so that they will follow him regardless of their lives, undismayed by any danger."

- If the ruler is exposed to insubordination, this ruler might commit suicide. ( see below). So soldiers must obey rules on the fields.

"14. Now an army is exposed to six several calamities, not arising from natural causes, but from faults for which the general is responsible. These are: (1) Flight; (2) insubordination; (3) collapse; (4) ruin; (5) disorganization; (6) rout.

[Tu Mu cites the unhappy case of T`ien Pu [HSIN T`ANG SHU, ch. 148], who was sent to Wei in 821 A.D. with orders to lead an army against Wang T`ing-ts`ou. But the whole time he was in command, his soldiers treated him with the utmost contempt, and openly flouted his authority by riding about the camp on donkeys, several thousands at a time. T`ien Pu was powerless to put a stop to this conduct, and when, after some months had passed, he made an attempt to engage the enemy, his troops turned tail and dispersed in every
direction. After that, the unfortunate man committed suicide by cutting his throat.] "

Conclusion : So  yes, Duke Su won the battle when he killed the ruler's enemy.
So the remaining soldiers either surrendered or retreated.
This is not a dream, he came back home to Xue Fang Fei. 

I hope this helps ^^

Without allies coming to rescue him, he would likely be captured and traded for benefits to the "defeated" Dai soldiers. 

I never thought of it this way or knew of this phenomenon or "code". 

I deduced my meaning of the ending by the flow of storytelling (death of JiJi and insurmantable enemy odds) as a slightly ambiguous sad ending and then the extra episode as a way to take it back and make it a happy ending to satisfy XueXiao fans. However, that SE seemed too abrupt and poorly handled as the screenwriters had been impeccable in handling the storytelling and adaptation, that even if it was a sad ending I found it hard to believe they would present it so poorly.

However, considering your point that once he killed the general the army would surrender (the rules they had followed earlier during the palace rebellion) it makes more sense.

The screenwriter wanted to convey the lost in battlefield and presumed dead that was the final plot of the novel: the hopelessness of waiting and surrealism of his return when there was no news for years. This explains why Wen Ji and Lu Ji had to die and why it looks like dreamscape.

Thank you, I think I finally get their intention. My faith in the production team is restored.

You are very much welcome ^__^

The era of depiction of the events is important and the millitary codes aswell.

 At a time when losing your honor could get you killed, I couldn't believe the narrative some viewers had  disregarding the ethic and codes making them believe that ordinary soldiers could take command and decide to kill an enemy when their ruler is dead on field. 

Soldiers obey orders they don't rebel or show insubordination. 

I felt compelled to make this point because a lot of people were fighting  over this topic.

This is my discussion about interesting points that i copied - pasted  here

There some tragic elements at the end of the drama that leans towards an open ending. So I understand why some people could think of it as such.
- The death of Wen Ji and Lu Ji,
- the men on horsebacks after the battle,
- the subjective camera when Xiao Heng gallops toward Xue Fang Fei .

Yes, I agree this is for easthetics . One could think about it as an open ending if we only consider these elements. However, as you stated, and I agree. Showing all that at the end of the drama adds a bittersweet taste to finish the show.
We could say that this leans toward an open ending because, as you said => it leave room for imagination.
This a manipulative strategy  writer / director use to add momentum at the  end of a movie.
Also, I totally agree with you: Bittersweet / sad ending are more memorable. So I get your point.

Yet, the subjective camera was used several time in the drama . So it does not add a meaning of bitterness or melancholy.
thus, when viewers understood this as a sad horseback scene I disagree.

If it was my choice.
The director could have let Xiao Heng reach Xue Fang fei under the tree.
But he did not. So=> room for imagination. I get it.

Nobody can change how viewers feel. I explained the logic of events as they occurred in an historical context. This set the basis of the
characters behaviour.
 I cannot discuss viewers taste for open endings. This undeniable.
I am going to copy past this on the discussion above, because yu opened my views on this topic, thank you.

A fellow MDL member spoke about a sad ending. Here is the post : 

Strong Spoiler caution  :

The original ending was not a happy one, Wang Xingyue released a clip where Duke Su died in the battle... There is a mess on weibo rn lol
It hurts to see this, be careful if you want to watch, if I knew I'd rather not opened the video ?

Wang Xingyue was eager to cut the CPF fans and his studio released the BE footage with the death of Xiao Heng in the battle.
The director Yu Zhen posted a weibo with some chat records where he defended his HE ending, and he said: "Today's incident really surprised me. I don't understand fan circles at all, and then suddenly the company's publicity team released some behind-the-scenes clips. I was a bit confused and severely criticized them, but they said it was because of the actor's highlights. I didn't understand this logic either. But these clips were added by the director, who wanted more options, and they weren't originally in the script.
In the afternoon, a lot of voices came out, and I got a bit anxious. So, I sent the Weibo posts I saw to the publicity team, asking them to handle it. As a result, I accidentally liked one of the posts. After being reminded by the publicity team, I immediately deleted it. Then, I happened to be delivering a bracelet to Xing Yue, and many fans were asking me about this incident. To this day, I still don't know what happened. Whose responsibility is it? Should the publicity team be fired or what? The actor is completely clueless; he's been busy filming and hasn't been checking his phone much. I need to think about how to handle this. Let's all calm down for now."

After that, WXY's studio posted an apology statement:
"Apology Statement:
Hello everyone, this is the Wang Xingyue Information Station.
Regarding the recent public controversy caused by the content released by our information station, the company has taken it very seriously and has initiated an internal investigation. The following measures have been taken:
The relevant content has been deleted; The publicity staff responsible for reviewing and releasing the related materials have been severely criticized and dealt with according to company policies.
All publicity personnel have deeply reflected and introspected on the mistakes made in their work, and we sincerely apologize to all the viewers who love "Mo Yu Yun Jian" and to the netizens who have been following this incident.
We once again thank everyone for their love of "Mo Yu Yun Jian," for their love of Xiao Heng, and for their support of actor Wang Xingyue. In the future, the information station will strictly control the content released to the public, improve the work review and supervision process, actively address deficiencies, and accept everyone's supervision. We hope to continue to accompany the Wang Xingyue Super Topic community with a better presence in the future!"

Atm the director Yu Zhen is trending at #1 and WXY at #3 on weibo, there's quite some chaos among the fans. ?

I know that in the entertainement industry/ movie industry, they produce alternative endings to keep the show secret. That way,  nobody knows how the show unfolds. Maybe there is a little bit of that too.
The staff should have not leaked the material.

Here I past an excerpt of a comment ( from @Regina de Sá ) about the last scene of the drama from and a message from the director of the drama Mr Yu confirming the Happy Ending. 

(@honeylemoncha):  " ahh today a friend pointed out a nice bit of connection between XFF's dialogue + visuals re: the ending that i didn't notice the first time! (spoilered below)

when XFF visits JYB (JL's father) after XH leaves for war, she tells him:
"If [XH] can't make it back, I'll tie my hair and be a widow for the rest of my life."

in the ending sequence she's wearing her hair loose <3 "

@Regina de Sá  : "Yes, I noticed this detail and commented here (MDL). There are many subtle details that reinforce the HE, for example, in the final scene XFF is in red, the Duke too. There's no doubt about that. The Double is an epic and has an epic ending, an epic HE. Not only that: this ending is not banal, it is not “sugar water”, it follows exactly the style of the entire narrative, the storytelling. A friend of mine posted here a letter from the director about the end. He didn't include the source, but this friend is very serious in his comments, so you can trust the letter:

"Yu Zheng: "Generally, pessimistic people will overwhelm optimistic people. People because they are pessimistic will chatter endlessly and want to prove they are right, while optimistic people don't care about arguments and live in their own world. So, I decided instead of me explaining 40 was HE, I would make 40.5, so everyone can see for themselves it was HE". "

excerpt :

 "- XFF's jade pendant ( it witnesses  XXF's love to her significant other. Her daughter wears it on the 40.5 episode so this proof that XH came back from the battlefield and gave it to her),"

This drama is so well written.
What I like about this drama is that even objects have their meanings and importances.

They are like characters, they speak the truth about characters intentions and feelings.
Like :
- XFF's jade pendant ( it witnesses  XXF's love to her significant other, her daughter wears it on the 40.5 episode so this proof that XH came back from the battlefield and gave it to her),
- Jiang Li's hair Pin ( the witness of Jiang Li's true identity . Thank to it, ,XFF could become Jiang Li),
- SYR's Hair pin (it witnesses the status of his relationship with Princess Wanning ),
- Emperor's gong (it strongers the bound between XXF and XH throught XH's background story: same story : save their father's reputation and /or life ),
- SYR's flute (it was the witness of SYR and XFF love story, then,  it  became the witness of SYR murderous intents, at the end,  the flute was the witness of his death and the same past love aria was played like a funeral parade leading him to his death down the ground)

And of course there are other important objects in this drama. It is delightful to remenber all their meaning.

This drama is so beautiful and meaningful (*___*)
I don't know if another one can surpass The Double ^^

Why not the other way round? The Sad Ending is the Alternate ending whereas our ending is the Original one.  (grin*) Hence, the director just wanted to continue filming that day when WXY was decked out so fine, with that jade pendant in his mouth. I know those Chinese directors love to film a couple of scenes of vomiting blood before they could be happy....

I think that is exactely what happenned. The director wanted to protect the drama from gossips  and / or leacking the ending. So he filmed a sad ending where the actor has blood in his mouth. This ending was not supposed to be leacked to the public. This is why the people who leacked the footage where scolded. 

The real ending is an Happy ending. 

American showrunner do it everytime : If my memory surves me right. At the last episode of the sitcom  "FRIENDS", the producer instigated the rumor that multiple ending of the show would be filmed. In reality only on was filmed.

And I am certain that he came out of it unscathed via cayenne pepper rescue method!!! If you saw Go Princess Go around ep 27 there is a famous" inside the tent" battle that you just have to see to know how Su gōngjué escaped clutches of death

Cayenne pepper rescue method  ha ha ha ^___^

I will have to watch this episode of Go princess Go because now I am curious. 

I am so happy Duke Su was not killed. First of all, he is a badass, it would be so unfair for him to die like that. Second of all, the guy had to keep his promese, we already witnessed the death of Wen ji and Lu ji. For him to be killed aswell would have been to harsh on us viewers.

Also, we know the drama likes to mirror its scenes with good vs bad. For example, SYR "killed" XFF in the beginning and hurt her where she was best at (her head/talents) and left her to die without looking at her. The mirror-scene is XFF "killed" SYR at the end and hurt him where he was best at (his hands/scholarly writing--that poetry on the wall, his art) and left him to die without looking at him.

So, there is a scene of Duke Su's father in armor fighting to the death because he had no hope. In the final mirroring scene, Duke Su fought to live because he had his wife to return to (pendant)

There are other mirroring scenes--the Crazy Princess taking away the "innocence" of SYR lost the most innocent thing she thought the Heavens had granted her--a baby Also, she was killed by the very thing she had corrupted.

The mirroring scenes of Jiang Li under her pear tree (her Li is pear) and finally, of XFF with her own tree, now back with her own identity.

So many more!


Cayenne pepper rescue method  ha ha ha ^___^

I will have to watch this episode of Go princess Go because now I am curious. 

I am so happy Duke Su was not killed. First of all, he is a badass, it would be so unfair for him to die like that. Second of all, the guy had to keep his promese, we already witnessed the death of Wen ji and Lu ji. For him to be killed aswell would have been to harsh on us viewers.

Yea, when the two Stooges died I was like "Journey to Love " slaughter again???? NOOOO!