Is it only me but Plex never worked stably on tvOS. The transcoding stops every 20 minutes and the movie freeze, forcing me to restart it. I suspect it's only with the mkv container and when using subtitles... Besides that, it forces you to transcode, and you need to subscribe to have native decoding. I use Plex for many years, even wrote and published plug-ins for it, but I could never convince me to subscribe. There are too many quirks that make me think let's wait until it works...