I do not have any Roku devices, so cannot comment on their behavior.
I have not noticed any issues with recognizing forced subtitles in the following clients: Android mobile & Android TV, Plex app for LG TVs, Plex Web, and Plex for Windows.
I’m running Plex Media Server 1.21.0.3608. It has no issues recognizing embedded or external subtitles.
Using the tools I mentioned, you can determine if subtitles tracks exist, if the forced flag is set, and modify the flags as desired.
You should also verify the Audio & Subtitle settings for your Plex account (Account Settings).
You can also try Plex Dancing one of the movies. That will force Plex to re-recognize the movie, removing any saved behavior from previous subtitle selections.
