Hello,
I've noticed than when the Apple TV selects subtitles automatically, there is a bug that causes the app to choose the wrong subtitles.
My account is setup like this :
For some reason, when I have English and French subtitles available for a file, Plex is choosing the French subs by default (which is wrong per my config).
But the worse comes when I then switches subtitles to English. I'm getting a mix between both subtitles files :(
I have no such issues on iOS.