I just made an MKV file for Pirates of The Caribbean - Dead men tell no tales (4K). The movie plays fine on the VLC media player and the Plex player on my computer, but not on the Plex server, it skips and shutters like in slow motion and out of sync. I have encountered this twice with Plex, the first movie was Guardian of the Galaxy: Vol 2 and now Dead men tell no tales. I have more than 300 (4k) movies and they all play fine using apple TV