i had those same issues with firmware 7.6. So its the hardware, not firmware, these units have bad hdmi handshaking issues when hdr is enabled. I was ready to return the device, but i hooked up the device straight to the tv and eventually back into receiver, to a different hdmi port. Turned off deep color off/on the tv, it eventually started working great like magic with 4k hdr working.
Its even better now with firmware 7.7 as no more audio chirps. But that 4.2.2 setting is unusable, 4.2.0 seems to work
the issues may have to do with the roku not turning off 60hz refresh rate after playing hdr?
I didn't have this issue with the last firmware. I did have the issue a few months back with one of the firmware updates however. I don't believe it's a hardware problem, it only happens to me with certain firmwares. It isn't related to HDR this time. Doesn't matter if I had HDR on or not it was doing it until I unplugged the HDMI cable and waited a bit then plugged it back in.
4:2:2 and 4:2:0 both worked fine on my set up except for this 7.7 release and one other firmware release, all the rest have been good in that area.
Also, The Roku is always playing at 60hz at all times, HDR or not.
I just went and woke up the Roku and it was back to flashing and audio going nuts again, remember I still had it set on 4:2:0. Unplugged the HDMI cable and waited, plugged it in and same thing. Unplugged HDMI cable again and waited a bit longer then plugged it in then it was happy again. I hope they fix this in another firmware release soon. Sucks having to unplug it like that and replug it in when I want to watch it.