I have a Sony X900E with a Sony DNR-1080 Receiver connected to it. All the receiver inputs are capable of moving HDCP 2.2 signals. Two of the TV's HDMI inputs can be "Enhanced", capable of displaying 4k HDR HDCP 2.2 images.
I recently bought a 2018 version of the Premiere. If I try to set it up using one of my AVR's "Enhanced" inputs, the Roku indicates I do not have a HDCP 2.2 input and reverts to 1080p. If I force thee Roku into 4K HDR mode, the screen turns black every few seconds and is unusable.
If I plug the Roku directly into the TV's "Enhanced" HDMI input it immediately shows that I do in fact have 4K HDR at 60 HZ.
I too have jumped through the troubleshooting hoops suggested by Roku to no avail. My firmware is 8.11 - build 6021-91.