The bug had nothing to do with forced vs. non-forced subtitles. It just didn’t select the correct subtitle language if you configured different languages for preferred audio language and preferred subtitle language.
Now, with the bugfix, everything works as documented here: https://support.plex.tv/articles/204985278-account-audio-subtitle-language-settings/
Automatic selection of forced subtitles should work as documented here: https://support.plex.tv/articles/200471113-configuring-subtitle-support/
But that is either still bugged or not designed to work when you have different preferred audio and subtitle languages. I’ve asked about the details here (in German).
