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.
Now, inexplicably, forced subtitles don't work. They are either on or off, but the forced flag makes zero difference.
For containers such as MKV which can have embedded subtitle tracks, the tracks can have a "forced" flag. This enables the effect described above.
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.
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.
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.
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.
I checked and re-set the forced flag for the forced subtitle tracks using mkvpropedit without success. VLC media player automatically switches the forced subtitle on that matches the audio language.
Enable or disables subtitles. Subtitle tracks with the "forced" flag are unaffected by this and should always display.
"Only Forced Subtitles" This mode will only pre-select subtitles flagged with the "forced" flag. It'll also obey the language preferences, of course.