I recognized yesterday that the Plex App of tvOS and iOS no longer shows forced subtitles.
My files are all MP4s with SRT subtitles.
If I play the same file on macOS with Quicktime or on tvOS via the Computer app the forces subtitles are shown.
Selecting in the tvOS player other non forced subtitles works (even without transcoding :-) ). Only if the subtitle mode is "automatic" where the forced subtitles should appear is not working.
Selecting the forced subtitle in the Plex splash screen of the movie before I start the movie has also no effect.
A few month ago this worked since years... Must be an update from the last months.
On tvOS 10 and 11 forced subtitle are broken. Must be a Plex issue.
Some Subler screenshots of a movie with forced subtitles: