Recently I have been noticing color banding / artifact issues on black screens whenever the Ultra needs to upscale sub 4k content to 4k. This behavior disappears if I change the roku to output 1080p instead of 4k and let the TV upscale. I've noticed this behavior across all apps whenever the video source is being upscaled to 4k. I've attached two screen shots for reference. Both pictures are from the end credit sequence in the Vietnam documentary on netflix. I've observed this behavior in uncompressed blu ray playback as well so I don't think it is an issue with the stream bit rate changing. It's possible to workaround this issue by leaving the roku in 1080p and manually changing it to 4k whenever I know I am watching a 4k stream but I would prefer not to have to be changing the output resolution all the time.
Great suggestion I wasn't aware that we could change the subsampling setting on the Roku. Unfortunately it didn't resolve my issue but it did lead me to mess around with the refresh rate. I found that if I changed the output to 4k 30Hz instead of 60Hz the picture quality cleared up. I'm not entirely sure if the output is actually 30Hz because my tv is still reporting a 4k 60Hz signal off of the roku. I tested 4k HDR content off amazon and it played as expected and 4k 60fps youtube content played back without any issues. It definitely felt like 60fps and youtube stream analyzer reported it. I kept it at 4:2:0 when changing the output to the 30 Hz setting.