I had the exact same problem as described in the first post. My setup is Pi 5 8GB connected to Denon AVR X2800HDAB connected to Sony Bravia XR55A80L. If the Pi source was selected/active on Denon while booting up Pi everything worked fine. As soon as I changed the source on Denon and then tried to get back to Pi I would not get video (No signal) in 50+% cases. Audio, on the other hand, worked (I could hear UI sounds).
That was the case as long as I had Pi connected to CBL/SAT or Media Player HDMI input on Denon. Moving Pi to AUX1 or AUX2 HDMI input (two of three 8K ports on Denon), for some reason, solved the issue. Switching to Pi source now works flawlessly.
I'm running Pi at 3840x2160, 60Hz with this added to the cmdline.txt:
video=HDMI-A-1:3840x2160M@60D,margin_left=0,margin_right=0,margin_top=0,margin_bottom=0In addition to that, I added the following to the config.txt:
hdmi_force_hotplug=1
disable_overscan=1