This sounds like the HDR10+ issue I have that was introduced with the ExoPlayer updates from in the Android Plex client from 8.28 onwards. More details can be found on this here Plex Player app on nVidia Shield is broken - #52 by BigWheel
It has been fixed in ExoPlayer dev branch but is not yet integrated into a stable ExoPlayer release or in the Plex Android client.
This is possibly fixed in the upcoming 9.6 that you can already side load from the linked thread as of a few mins ago.
