Changing the audio encoding (EAC3, AAC, DTS, etc...) doesn't seem to make any difference
The Plex Server will transcode AAC, DTS, and EAC3 5.1 audio down to AC3 5.1. DTS stereo is transcoded to AAC stereo by the Plex Server. The Roku will still directly play the video but the plex server will transcode the audio.
Thus, using EAC3, AAC, and DTS 5.1 will all result in the Plex Server transcoding the audio. Of the modern audio codecs, only AC3 5.1 audio is direct played on the Roku by Plex.
For whatever reason, if the Plex Client on the Roku shows the container is mkv when the audio is transcoded and the HEVC video is direct played, the Roku will play the video with HDR, no matter if the source is SDR or HDR. This is a result of the issues I described in my prior posts. The 4800x Roku forces HDR anytime the Roku believes the container is an mkv, which is what the Roku sees every time EAC3, AAC, or DTS audio is converted to AC3 and HEVC video is direct played from the Plex Server.
It does not matter if the original container is an mkv or not, if the source video is HEVC and the audio is transcoded, the plex client on the Roku shows the container is an mkv and the video will be played with HDR.
In my testing, converting the audio to AC3 and moving the audio and HEVC video into an mp4 container via ffmpeg allows for a complete direct stream of audio and video from the Plex Server, and the "always on" HDR issue stops. If the same audio/video combination is in an mkv, the "always on" HDR comes back right away.
Again, if Roku fixes the "always on" HDR issue with HEVC/mkv files on the 4800x, these related audio conversion/direct playback HEVC issues will be solved also.
Lastly, one of the test videos I uploaded earlier that works correctly has no color range listed yet, it plays back perfectly. Why? Because it's HEVC in an mp4 and it's completely direct played back, both video and audio. i.e., https://mega.nz/file/lfxWzRaS#8SlZl9DMYNaLvEiYPRSafrjzi263yHzmsaRf4Q0mBeo
I am 99% certain this issue has nothing to do with color ranges and everything to do with the way the Roku handles HEVC in an mkv container, which is also triggered by the Plex Server transcoding EAC3, AAC, and DTS audio with HEVC video not in an mkv container.