Makes me want to stick with Oppo so I can just run the audio to the receiver with the 2nd hdmi.Eureka, I have discovered what causes the banding. I hooked up the Oppo directly to the B6 and the banding is gone with 4:2:2 10-bit.
This means it is the Marantz SR7010 that is messing with the signal, even though I've turned off all video conversion/scaling/processing (to the point where I can't have onscreen diagnostics overlayed on the movie).
When the SR7010 is in the chain with 4:2:2 10-bit input, the Marantz shows color space is BT.2020 (like normal), but pixel depth is shown as "--:--", whereas when I make the Oppo output 4:4:4 12-bit into the SR7010 it shows as 12-bit.
At first I didn't suspect the receiver, because I've seen people with different brands of receiver report this problem, but Marantz and Denon are owned by the same company and likely share a lot of hardware/software.