So, I don’t really care about this but as it’s apparently unexpected behaviour, I thought you might want some logs to look at.
I’m playing the test clip called The World in HDR (HEVC version). It plays fine at 4K, but if I switch resolution to 1080p then all of a sudden I’m seeing the kind of posterisation/banding that I associate with 8-bit output instead of 10-bit. The output signal is still 444 10-bit, but the banding is unmistakeable.
Here are some logs. Playback starts at 2160p then I switch manually to 1080p later. Other methods for playing it at 1080p (such as de-whitelisting 2160p modes or setting Adjust Refresh Rate to off before playing) produce the same effect.