I don’t think the novel ending would have passed censorship, so the drama had to make changes.
"In the novel, they continue living their life as emperor and empress, have children and stay together many years. Xiahou Dan remains weak due to the poisoning and dies before Wanyin. Wanyin rules alone, then abdicates, travels the world as she had always wished. She dies old with no regrets, just to wake up on the subway. She then search for Zhang San and finally find him as a CEO of a company. He explains that he came back in 2016 as a teenager and since then he's been waiting for her return. They book a room (^__^) and swear to live another life together. " taken from another comment
FR like i am sooooo confused with the current crown prince and zhang san, there was this scene before they met…
zhang san is the xiahou dan real name in the real world. basically young xiahou dan and current crown prince are played by the same actor (they look the same because they are father and son lol). just bear in mind that we only see the young xiahou dan in zhang san's diary, during the actual episode it is the crown prince (his son)
taken from another comment