I don't have HDMI 2.1 compliant devices so I don't know if this is the same or isolated issue.
I assume that this dropout doesn't happen on 4K60? If so, it could be the cable not having enough bandwidth or maybe a firmware issue on the AVR.
I also have quite an eccentric setup, since I'm using the PC with an Ultrawide (3440x1440) 144Hz monitor. The AVR does not support that resolution and refresh rate so I had to use an "Extended Display" option, with the DisplayPort on the GPU to the monitor, and the HDMI on the GPU to the AVR. The display is arranged like so to prevent mouse cursor from entering the second screen.
Display 1 (DP 1.4) gets 3440x1440 144Hz
Display 2 (HDMI 2.0) gets 1920x1080 60Hz
View attachment 197827