In my experience, after ripping a DVD or Bluray, none of the subtitle streams is usually tagged.
Of course this is highly dependant on the software you are using to rip the discs. Some of them don’t even rip the forced streams without you explicitly telling them to do so.
I made it a habit to check every ripped file in MKVtoolnix and add the titles and the “forced” flag if applicable.
The Plex subtitle mode “Shown with foreign audio” is dependent on the “forced” flag. If none of the subtitle tracks in your file has it set, Plex cannot know to enable the right subtitle.
MKVtoolnix has a “header editor” which can add titles and the forced flag without the need to remux the whole file.
