I recently bought Roku Ultra 2020 to enjoy 4k HDR 10 Dolby Atmos content.
I have Denon receiver (AVRX3700H) and Sony projector (VPLVW295ES)and combination of both should support 4k HDR 10 Dolby Atmos. I tried the steps mentioned in one of the community post to verify HDMI 4K capabilities from Roku setting option in menu. It confirms the sink capability to be 4k HDR10, but when I select that to set as display nothing get display on projector screen. So after the time out screen, display sets to 4k 8 bits resolution.