HOWEVER, if I then load that MKV file into mkvtoolnix's header editor and look at the subtitle tracks, NONE of them have the "Forced display" flag set.
I'm not sure if I'm 100% clear on when to use default vs the forced tag vs no track flag for subtitles in an mkv. I think concrete examples might best illustrate proper usage.
Enable or disables subtitles. Subtitle tracks with the "forced" flag are unaffected by this and should always display.
For containers such as MKV which can have embedded subtitle tracks, the tracks can have a "forced" flag. This enables the effect described above.
Now, inexplicably, forced subtitles don't work. They are either on or off, but the forced flag makes zero difference.
Forced subtitles appear in many movies and only provide subtitles when the characters speak a foreign or fictitious language, or a sign, flag, or other text in a scene is not translated in the localization and dubbing process.
I also do no get forced subtitles on any MKV movies I highly doubt all the movies I have tried don't have the forced flags set properly.
"Only Forced Subtitles" This mode will only pre-select subtitles flagged with the "forced" flag. It'll also obey the language preferences, of course.
The Dune does not honor the Forced flag in the subtitle streams. This causes problems with movies like District 9 that depend on subtitles being enabled. To fix this problem, run this dsf file on your Dune. MKVs that have forced subtitles will have a brief hiccup when they begin to play.
Currently if a MKV video with an embedded subtitle having the forced flag is played, MPC will ignore the meaning of the forced flag.