I am seeing this exact same issue with my 1080p HEVC content. The strangest thing is the max resolution on all of my TVs is 1080p thus, the roku should never be attempting to output anything to HDR. Using my 2019 roku ultra, no problems at all, but the 2020 roku (4800x) is severely broken.
I also believe a new profile needs to be written for this new roku. I noticed HEVC content missing the ctts atom has broken playback. I have not seen that on any other plex players that I have as my other devices correct for the missing ctts atom automatically. The 2019 roku ultra handles HEVC content missing the ctts atom, but the 2020 roku stutters to the point the video is unwatchable, unless I force the plex server to transcode the video.
For the time being I have moved the 2019 roku ultra to my main television and the 2020 roku ultra has been relegated to spare tv, to reduce the need to transcode video to work around the HEVC/HDR issue on the 2020 roku ultra.
