Have you tried toggling the “Use Hardware Decoding” setting?
Settings → Plex for Windows → Player → Use Hardware Decoding
Hardware decoding may improve video playback performance. Disable this setting if you encounter unexpected playback problems.
Edit: Since v1.26, the app supports using a mpv.conf file. This may let you fine tune the app to work with your display.
See the following for additional information:
C:\Program Files\Plex\Plex\mpv.conf.md ← adjust accordingly if installed in a different location.
Plex Documentation → Plex Media Player → Advanced Configuration
