They’re all .mkv files. The issue isn’t that there are no subtitles, or that the subtitles are burnt-in to the video, the issue is that the first subtitle track is getting marked as ‘forced’ when it shouldn’t be.

I’m not sure how to stop the DAC from shutting down when Mint goes to sleep, but maybe you can avoid the problem by making Mint not go to sleep in the first place? There’s an option for it in the System Settings program, in the “power management” menu (under the “Hardware” heading).

Try doing that and see if the problem keeps happening. If it keeps happening, the DAC shutting down was probably caused by something else.