I am copying this is from the Plex forum as this belongs on this forum also.
I had a major breakthrough in solving the mystery of the 4800x and HEVC video that I believe is worth a tag.
A user on the Plex forum had me test his handbrake encoded MKV files and they played correctly on the 4800x. That lead me to reencode my test file using Handbreak to make to see what would happen. To my shock and amazement, it played perfectly on the 4800x.
My test video is AAC mono, originally shot on an iPhone 6s. This is important as this means the plex server will not attempt to transcode any part of the newly constructed MKV files.
When I add the new MKV file with the MKV container that has just been built by FFmpeg to Plex and try to direct play it on the Roku 4800x, now I am seeing the Roku 4800x apply BT.2020 color to the video despite the fact that the OG version with the container encoded by HandBreak plays the video back as BT.709.
This explains all the issues with the plex server transcoding multichannel AAC also, as 5.1 channel AAC audio is transcoded to 5.1 AC3 and merged with the original HEVC video in a new MKV container when transcoding. Since FFmpeg is adding/missing a flag, or something else, compared to Handbreak in the newly constructed MKV container, the video is played back as BT.2020 HEVC despite being coded as BT.709 HEVC.
To me, it’s underly mindblowing that a new Roku would not work better with the current FFmpeg release.
In any event, the cause of the 4800x issues has been narrowed down. Something with the 4800x does not process 8-bit, HEVC, MKV containers encoded with the latest FFmpeg properly and this is why we are seeing BT.2020 color when we should be seeing BT.709 on the displays.
Are they fixing this? I just had this problem today and it was driving me crazy. I bought a new Roku specifically to avoid having to transcode my media, but now I have to transcode it to avoid everything looking red and saturated.
I use Plex. Thought it was a Plex issue at first. Unfortunately, my files are MP4 with AAC audio. So it looks like they're getting re-muxed by Plex into an MKV to handle the AAC problem (by transcoding to AC3). Not sure if there's a straightforward solution in this situation.
The problem I'm having is that my video files are MP4, being played through Plex on a Roku Express 4k+. I have two files that are both HEVC video with AAC audio. One has the color space issue where the colors are all red/saturated and one does not. Anyone have any idea what is going on and how to address it?