It seems like the Plex developers changed something on how forced subtitles are handled. There is now a new option in the user settings on how forced subtitles should be selected.
Sadly I don’t see an immediate way on how the new available options help in my situation. Also, the documentation hasn’t been updated for the new options yet.
And I can’t test it myself to find out if the options help because the whole subtitle selection seems to be broken in PMS 1.15.3.876, see [BUG - PMS 1.15 regression] Automatic subtitle selection broken