I have an nVidia Should Pro 2019. It supports HEVC. However, the native Plex app does not. If I try to play any HEVC videos from my Plex server, instead of it playing as a direct stream, it has to transcode.
If I instead install the Plex plugin through Kodi, it plays as a direct stream perfectly fine.
Therefore this is a limitation of the official Plex app itself.
Will HEVC support be coming to the official app anytime soon?
Interestingly, with my new TiVo Stream 4k (Android TV) box, i was also unable to play HEVC content until I hooked up my HDMI via my audio receiver (rather than straight to my 4k TV which is how I originally had it). Now, be taking the signal through the receiver, it will play all my 4k movie content in original quality (no errors, no transcoding).
I can direct stream every single file I’ve thrown at my shield in Plex. 4k H.264, HEVC, HDR, Atmos, True-HD, DTS-X etc. I have to believe that there is a setting somewhere that is incorrect on your end.
I believe the plex player has improved a lot. I have an old nexus player and I never expected it to play h265, until yesterday I manually updated the plex app and it’s doing it now. I would suggest you check your settings.