Why do so many Chinese short dramas have music added on top, sound cutting out from the official channels?

It's like they don't own the media they are posting, but then it makes no sense they would do that.

https://www.youtube.com/watch?v=QurgSer4V0Q

For example, has music playing over 28:08 which is louder than the original music or the voices.

Or the sound suddenly cuts out for no apparent reason.

Is there some restriction, etc on the official channels to post the full content of the movie?

I don't watch much on youtube, but I have encountered similar strange stuff where audio is simply cut out, even the dialogue. I assume it's because some song was playing.

It's most likely because of rights issues. Perhaps the rights holder of that song only licensed its use within China, and not outside of China. So by putting it up on youtube they are breaching that license, thus the makers of the shorts and dramas simply cut it out or in this case play over it with something else.

I have my doubts because you have dramas like this: https://www.youtube.com/watch?v=-T0KvIc9wZY

Where Tencent is run by the Chinese government, and they have to have the voices as the official studios for the sound mixing, and it still ends up with sound like this where the voices are distant. Paying for a license for overseas wouldn't be a big deal. Asking for new sound mixing wouldn't be a big deal either. There must be a reason for it.

Change your VPN to China or Hong Kong seems to rectify the issue. Reload the video. Let it stream in for a few minutes. YouTube seems to be using some sort of real time scanning but it's frightful! AI maybe? Yes it's effective at removing the un-copyrighted music but it's constantly dropping dialogue or reducing dialogue volume. Leaving no ambient sound so dialogue sounds like it's in a padded room or metallic echo sound. It's just awful and if I was the uploaders I would go YouTube for loss of viewership as the software seems to interfere regularly with the other audio. 

I think copyright issue outside of China. I think That's why they stopped upload on Youtube and have their own platform.