Flagging subtitles in M2TS as forced?

[Asterra]

I like to use a media player for watching movies since, unlike with any Windows-based solution, you get a 100% reliable 23.976fps (or whatever). But the Popcorn Hour A300 I use has a known-only-to-me (because nobody else has noticed it) problem where the audio goes out of sync over time in most MKVs, equating about 150ms every hour (regardless of whether a given chapter has been deliberately skipped to). My solution to this is to pipe the MKVs through TSMuxer to generate a functionally identical M2TS file which plays without issue in the Popcorn Hour.

But every once in a while, I get a video with subtitles that need to be on by default, and it's an embarrassment. This time I wish to correct the problem. I already used "Subtitle Edit" to convert the internal .sup into a .srt because TSmuxer hasn't a clue what to do with .sup files. Unfortunately, TSmuxer doesn't have any "forced" flag option.

I thought I would solve it by converting the resulting M2TS back into MKV, forcing them with MMG, and converting back to M2TS, but it looks like this process magically converts the subtitle track into something which TSmuxer again doesn't know what to do with.

After much Googling, I am stuck. I tried my best. Help. ;p

[Selur]

Are you sure there is a flag to set a subtitle stream as 'forced' in m2ts? (last time I checked there wasn't, but that was a few years ago)