Other users do RPi -> AVR -> TV without problems, so I think your AVR interrupts HDMI connection. Try RPi -> TV -> AVR instead.
If it has been working before, better HDMI cables might help. Maybe volume change increases data rate, and one or both of your HDMI cables can't handle that data rate. HDMI 2.0 or 2.1 standard cables work fine for RPi 4.
At config.txt you can try hdmi_safe=1 for maximum compatibility among devices.
Sorry to hear that. At least your cables are now conform to 4K.
Probably your former A-400 had a lower data rate, because it wasn't a 4K device. I think we now have isolated the AVR as the trouble maker when using higher data rates.
The only advice I can give is to order even more stuff. 
The second HDMI port (HDMI-1) for audio doesn't work. Get an HDMI audio extractor to connect your AVR separately.
Little update:
I tried the kodi nightly build, and switched the dtoverlay from vc4-fkms-3d to vc4-kms-3d-pi4 and the volume problem is solved. So problably the error is in the vc4-fkms-3d driver or it's missing some parameters for this setup to work.
Leaving the new pi4 kms driver is not an option at this moment, because the picture is constantly freezing . And I see a lot of alsa audio errors in the log. 1 problem solved, 1 new problem created 
You probably found a way to reduce HDMI data rate by using this driver. Keep my HDMI audio extractor advice in mind when searching for a final solution.
I'm not sure whether moving pictures really increase data rate on HDMI.
To check my theory, you could reduce the resolution for a test. This will definitely reduce HDMI data rate.
If you have audio pass-through activated, deactivate it for another test.
OK, here is my next and final theory. People often believe that HDMI pass-through is a simple in-and-out process, but that's not right. Your AVR has to analyze incoming HDMI data to find relevant commands. My theory now is that RPi's volume change data are challenging for your AVR's processor. Probably so challenging that pass-through of video HDMI data will be dropped for the period of volume change.
However, my advice stays the same: get the extractor. Then your AVR just has to deal with audio data.
Fixed.
Used the nightly build 22-05 noobs (Index of /) (LibreELEC-RPi4.arm-9.80-nightly-20200522-76f70e9-noobs.tar)
replaced rpi4 folder in official noobs release (Download NOOBS for Raspberry Pi)
On command:
mount -o remount,rw /flash
changed dtoverlay to dtoverlay=vc4-kms-v3d-pi4 in /flash/distroconfig.txt
On libreelec interface:
Changed audio output device in libreelec system config to : ALSA: vc4-hdmi,MAI PCM vc4-hdmi-hifi-0 S/PDIF
enabled audio Passthrough for all
No freezing, no hdmi signal drop by the avr...
Hopefully Libreelec 10 will use this by design
