SOLVED: Multiple simulatenous subs (same file) not staying on playback

[TheDreadPirate]

If it is direct playing I don't believe changing the burn in behavior does anything. I believe that only changes the subtitle behavior if there is video transcoding occurring.

Which is another test you can try. Set the burn in behavior to "always" and select a bit rate lower than the video's native bit rate to force transcoding. Do the subtitles behave as expected?

What we're testing is if the client side subtitle rendering is the problem, which is likely and something I see a lot with ASS subs, or if the subtitles themselves are the problem.