Wouldn’t have a clue. Haven’t looked much into codecs. It only stops when using the subtitles though.
Took the file apart in MKVToolnix to see if there was anything there. The subtitles are either .ass or PSG. Which I then exported to .srt since Safari wouldn’t pull them directly from the .mkv.
The exported .srt is what plays nicely in Plex Media Player for Mac but not in browser