I am having a similar connection issue as you. Here is my setup
Epson 505-UB, 50ft HDMI 2.2 compliant cable, Marantz 8802A, Apple TV 4k.
For me, I can get no communication to work, at all, through the Marantz into HDMI 1.
I cannot get any communication to work, at all, with the Apple TV 4k directly driving the 50ft cable into HDMI1.
I can get my Oppo Blue Ray play which can upscale to 4K to play through the Marantz and into HDMI1.
Here's what's weird now.
I can get the Apple TV to communicate to the 5050 on HDMI2 input. It will not work with HDR 4K but SDR seems to work. This works both directly driving the long cable and through the Marantz 8802A.
I have tried two different AppleTV 4k units so either that source is underpowered or there is a problem elsewhere.
I am suspecting a problem in the 5050 on HDMI1 at this point and my next step may be to get another unit from Best Buy through Magnolia.
(I have not tried the firmware update)