Note that this is not the same issue I was reporting here:
Things have got worse since yesterday! Yesterday, a 4K/60fps/HDR clip was playing fine at native resolution (but was giving me posterisation/banding when downscaled to 1080p). Today, even at native res, all of my 4K/60fps/HDR videos are being output with an 8-bit signal rather than 10-bit, with consequent banding artefacts.
This may possibly be as a result of my going onto the staging repository yesterday. But I guess it’s also possible I’ve screwed up a setting somewhere while testing something else.