Same issue for me. Tried several different 4K HDR movies on my Xbox One S and some would not play at all and some would play for 30-40 minutes and then start buffering. After skipping 20-30 seconds playback would resume for another 30-40 minutes when it started buffering again.
I just cancelled my monthly plex pass subscription with the explanation “Technical difficulties” and as a comment I wrote that they should fix the Xbox app because otherwise it’s just worthless for me.
