This is the file you linked in another thread which I downloaded. I remuxed them into an mkv container which worked perfectly fine. I was able to change the quality of the demo in its new container. So I assume the file is just not perfectly suitable with Plex or at least this Plex version. I don’t think we have to look into it any further.
Back to the actual topic: so, what’s different to to the 1.29.2. Plex version you provided? It looks like it has not been published as container image. Is this an internal version with specific settings or did someone simply break HDR transcoding in 1.30.x and the verison you provided is the last working version?
This thread once linked to the 1.29.2.6273 thread which seems to be the verison you linked here as well. Unfortunately, since October the same thread has been updated to 1.30.0.6359 which didn’t work for me in terms of HDR → SDR transcoding.
