Are you sure that's the same thing? That user never said the subtitles aren't showing.
It's the same thing. I'm sorry that I didn't recall it right yesterday. After testing again this morning, I remember that I merge subtitles with my mkv just to make my shield tv to direct play. No need to push to resolve this, because we can find an easy workaround.
And there is another mistake I made yesterday, which is that the Android mobile synced content bug is not related to external or embedded subtitles. It's just that one of my synced file can show external subtitles while others can't show any subtitle. I thought if I figured out the difference between the one working file and others I would be able to help you resolve the issue. But every difference I tested does not change the thing. Since you already have it resolved, I don't need to find the difference any more.