Yesterday I was wathing a movie that has vobsub (sub/idx) subtitles but the movie lost the titles after 22 minutes. It didn't bother me much because the movie had good enough .srt titles so I switched to them and continued.
I then started to watch another movie which also has vobsub subtitles and it lost the titles after 35 minutes and it didn’t have synced .srt subtitles. I then started to find out what is going on.
First I checked the vobsub subtitles with VLC (2.2.4 on macOS 10.12.2 (Beta)) and the texts were fine. Next step was to play the movie on Plex on iOS (4.5 on iOS 10.1.1) and movie lost the titles exactly at the same point as AppleTV. Also tried to play the movie on the web interface and it also lost the titles at the same point.
Last step was to try to play with Plex Media Player (1.2.0 on macOS 10.12.2 (Beta)), and to my surprise the titles were fine.
How to debug this problem, I haven’t found any settings that could affect this behavior.