I encountered the same issue yesterday when trying to play a 1080p HEVC file on a Samsung Galaxy Tab A9+. Downloading and watching the file locally with a different player (VLC) worked fine. I assume the plex app / plex server does not etimate the tablets capabilities correctly and forces a transcoding to H264 which hinders a smooth playback.
Same file plays correctly (no transcoding) on an iPad Pro 10.5" (2018) and a Samsung Galaxy Tab S6 lite with direct play.
I don’t have the detailed specs at hand at the moment, but happy to provide them if needed.
