On AppleTV (at least) when I play a video playlist and have subtitles turned on, the first plays w/subtitles, and the second, third, etc the subtitles do not play - EVEN THOUGH subtitles are chosen in playback settings->subtitles.
If I do playback settings->Subtitles->click on the subtitle name (unknown/mov_text) in the second/third, etc video, then the subtitles appear.
BUT it disappears on the next one.
This does NOT happen if - on the server - I force subtitles on using Plex->dashboard->Settings->languages->subtitles->Always enabled.
This is just a guess, but maybe it’s because each video has its own subtitle file. If the Apple client is set up to specify a file, then that file won’t work on the next video. And maybe the Plex server just serves the correct subtitle file with the correct video, so if it’s turned on server side, it works.
If you are selecting a subtitle during playback of a video (or on the preplay screen of that item), you are selecting it for that particular video only.
This setting is never used, except when the server is not signed into a plex.tv account. (which is a rather rare use case)
Verify that you have the same language selections made under https://app.plex.tv/desktop/#!/settings/account - AUDIO & SUBTITLE SETTINGS
These settings are specific to the user account. If you are using managed users, you need to define them for each user.