I started having a problem with forced subtitles, too. Forced aubtitles showed up just fine in Plex a few months ago but now the same files don’t automatically show them.
One WIERD thing is that if I go to the subtitle menu while playing and reselect the currently selected option (in my case OFF), the forced subtitle shows up. If I change it from OFF to anything else it also shows up. I can duplicate thus on all my MP4s with forced subtitles. Seems like toggling any subtitle option causes it to suddenly recognize the force subtitle.
Again these same files worked with forced subtitles previously and still work in QuickTime and VLC on my Mac, as well as the Computers app on the Apple TV.
I’m using the latest Plex Media Server and tvOS (including PlexPass betas) with no luck on an Apple TV 4K.
Please help.
This started prior to the latest beta but I can confirm it still happens in this beta.