I have a lot of local extras I’ve added to my library. Some of them have subtitles that I would like to enforce. The subtitle has been added to the files with the “forced” flag, and my subtitle settings in PMS are set to “Shown with Foreign Audio”. With regular content, this will force subtitles. However, with extras, it does not force them. Shouldn’t extras be handled the same way?
There is a ‘forced’ flag in media files.  PMS obeys this.    There is also the  Movie.ISO.forced.srt method (sidecar)  where ISO is the 3 character language code .
Please take a look and let me know if this doesn’t address it.   Also, please give me a more specific example so I can more readily follow along.
https://support.plex.tv/hc/en-us/articles/200471133-Adding-Local-Subtitles-to-Your-Media
Edit:   Local  extras do not use ‘sidecar naming’.  This is only for Movies as I have shown.    I apologize for any confusion / ambiguity .
Subtitles do not apply with Local Media Assets .   If you wish to use them, they must be contained within the extra itself.
Thanks for the reply! My subtitles are contained within the file, but the only way I can get them to display is by manually turning them on after I have started playing the file. To recreate this, create (or find) a file (I’m using .mkv, but I don’t imagine that matters) that has a subtitle file contained within it, and make sure the subtitle is “forced”. Then add this as an extra (mine are deleted scenes) and scan it into Plex, and make sure your server settings for subtitles are “Shown with Foreign Audio”. You’ll find that Plex does not display the subtitle, despite the subtitle being forced.
Edit: I’ve also found that, if you enable the subtitle for an extra, it will remember it for the next time you play it. However, this is reset when the library is rescanned, so it’s impossible to get an extra to always play a subtitle.
I think I’ve figured it out! The server won’t respect the forced subtitles if they’re not in your preferred language. The subtitles on my extras didn’t have a language set, so they were “und (undetermined)”. Once I changed them to english in mkvtoolnix, Plex automatically displayed them. Hoozah!