I suspect the problem is on the HDMI board of the receiver. It sounds to me as if the repeater architecture on the board is not functioning properly.
I would try a reset back to factor defaults. I would attempt this at least three time before you accept defeat.
Also you might try another TV if you have one.
How long is your HDMI cable? Issues can arise with these 4K systems if the cable is a passive one longer than 12 feet.
If the cable is less than 12 ft. and a reset does not solve the problem, I suspect your receiver needs a new HDMI board. If the receiver is out of warranty, it may not be worth it.
That is a receiver at the low end of the food chain, at just below $500.00. In addition Pioneer/Onkyo are close to bankruptcy. That outfit have a long history of HDMI board problems. From a quick search it seems they still are. I found reports of customers having multiple HDMI board failures with that receiver.
I know it is not very old, but I would strongly advise against spending any serious cash on that receiver.
If it comes to replacement, then I advise moving a long way up the food chain, so to speak, for a 4K Atmos receiver.