Similar to andirama , my issue appears fixed once I lowered the refresh rate to 30hz OR exclusively use the AUX1 or AUX2 ports.
If the resolution is set to 3840x2160@60hz, the Pi works fine if booted while connected to any of the Denon sources (CBL/Sat, Media Player, Blu Ray, Game, AUX1, AUX2) until the Denon is powered off. The only sources that work with the Pi when the Denon is powered back on are AUX1 and AUX2. As far as I can tell, there is nothing specific about the AUX1 or AUX2 ports on my specific x3800 Denon model and furthermore all of the ports work as expected when a Windows 11 PC is connected using the exact same cable on those sources and even at a higher refresh rate 3840x2160@120hz.
A simple test I found is that if I boot up the Pi and get audio/video via any source, I can unplug the Pi from that source and re-plug it into the same source or any other source and it will not give audio/video unless I plug it into AUX1 or AUX2. Just for sake of example:
Boot from Game = works fine
Unplug from Game and re-plug into Game = no signal
Move cable from Game to Blu Ray = no signal
Move from Blu Ray to AUX1 = works fine
Move from AUX1 to Game = no signal
Once I drop the refresh rate to 30hz, it works fine across the board and appears to work fine after the Denon is powered off / powered on or I can leave it at 60hz if I use one of the AUX1 or AUX2 ports. I need to give it some time to see if it is permanent but the above testing is 100% repeatable on my unit.
One more note for the rest of you Denon owners, I did get bitten by the default "Enhanced" setting for Video which prevented me from getting to 4k@120hz on my Windows machine until I set it to 8k Enhanced. It might not have anything to do with this but it definitely restricts your input even if it is capable