Apple TV 4K 3rd Gen
My Apple TV 4K 3rd Gen flashes white on the bottom third of the screen. This was happening on our 1st Gen, so we bought a new one, got a new receiver, and it’s still happening.
Apple TV 4K
With this behavior, I would suspect the HDMI circuitry behind the HDMI port on the TV. Have you tried connecting to a different port?
Please also test connecting the Apple TV box (and receiver) to a different TV.