Yeah, I encountered this problem too when I had to split apart the movies in my library about two weeks ago.
I find that once the split has been made, the extras won’t be detected if the movie files are in the same directory.
You need to create a directory for each movie file and make a copy of the extras that you will put in each of these directory. After that they will be detected again. Alternately, you can also just keep one copy of the extras and choose which version of the film will display them by placing them in the relevant folder.
It’s not great, and it does look like a bug to me.
