OK, I have just completed testing on my Ultra 4800. I have to provide a caveat that I am a Roku beta tester, and I'm not permitted to discuss anything about features in the unreleased OS versions, so I can't even tell you what version I'm currently using. Also, I have Plex Pass, so it's possible I have a different version of the Plex channel than some here. I can't say if that might make any difference.
With that in mind, I can say that I could find no red push anywhere when playing H.264/H.265 content. Most of my HEVC content has HDR, but I do also have some test clips with H.265 and no HDR. I tested both with the Roku Media Player and the Plex channel. Streaming from my media server, the audio is transcoded from the lossless codecs all of my UHD movie rips contain, but the video is passed through without any modification. My TV still triggers the HDR/Dolby Vision tag on the appropriate video content. But I also have some H.265 content on a USB drive, which plays through RMP without an issue. Again, no red push visible.
This is using a Roku Ultra 4800, passing through a Yamaha AVR (with all video processing turned off) and displayed on an LG Nano85 TV, which is HDR/Dolby Vision capable.
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