Hi - for my many music movies, I have edited in the titles of each Chapter, using MKVToolNix.
Mostly, they work perfectly. However, sometimes, a movie that had been perfect, sometimes has a 1-2 minute error. For example, in 'Moulin Rouge', One Day I'll Fly Away should start at 42.42,56, and it does (when selected via Apple TV or iPad Plex Apps) just after I've loaded it to my Plex library. But sometimes when I try later, even just an hour later, it starts at just after 44 mins, when selected.
In another movie, Chapters sometimes start a minute or so early.
When this happens, I use MKVToolNix to check the Chapter details, and they have not changed. The only way I can fix the problem is by removing the .mkv file, re-scanning the library, cleaning the bundles, emptying the trash, re-loading the movie and re-scanning the library.
Then it's fine again - for a while.
Any advice would be appreciated.