noggin thanks a lot for testing, this is in line with my suspicion (video driver using the wrong transformation matrix). Not 100% sure why 10/12bit output isn't automatically enabled, could be that we need to explicitly request it.
Regarding subsequent playback issues: Which testbuild did you use? The one from the first post (from Feb 5) or the one from here RE: RPi4 testbuild with HDR support (Feb 10)? The later build contains some important fixes that may be related.
If you got that issue with the newer build please post a log (ssh in, run "pastekodi" and post the URL) and ideally a link to a sample file so I can try to reproduce that locally.
so long,
Hias