It might be the amp. I heard that it needs a mod to prevent issues like what you are describing if it hasn't been modded already.
Issue is not the amp. I've swapped good-working channels with the CC and the issue follows the CC. Also, regarding issues with this amp, you may be referring to the link below. This issue was resolved some months back, and isn't a dropout issue but rather a low level hum/buzz issue. Not having this issue on my unit, guessing this simple fix is already in the box (haven't opened it to confirm).
We found the issue. The M8125/M8250 was designed to be driven by a balanced input signal, and no provision was made for an unbalanced input. Certain cables and adapters used to connect between unbalanced (RCA) sources and the M8125/M8250 can result in a low level buzz in even numbered...
www.audiosciencereview.com
Did some further troubleshooting:
1) First, streamed Apple Music over the ATV4K Music app, center channel had no dropouts
2) Returned to last known programming with this issue (MAX app, Righteous Gemstones). Within minutes I reproduced the CC dropout
3) Tried all the various Marantz sound options (Multi-Channel Stereo, Dolby, etc.), to see any of those might work. None worked
4) Entered Marantz Setup menu, specifically Speakers/Manual Setup/Levels/Test Tone --> CC tone works fine (this was a relief - thought possibly there was a speaker connection intermittency, and these are KEF in-walls, very heavy and not friendly to remove)
5) Returned to MAX, CC still dead
6) Fiddled with ATV4K audio settings (Dolby Atmos, etc), none made any difference
7) Back to MAX app - noticed the video was really jerky, stuttering along. Made me suspicious of the ATV4K and/or the MAX app
8) Restarted the ATV4K - CC still dead within MAX app
9) Deleted the MAX app (full delete), reinstalled it
10) CC audio restored on MAX app - ran it for an hour without issue
Doesn't explain audio failures on other apps, nor on the SpectrumTV cable box. Seems like there is something in the HDMI signal coming into the Marantz that it sometimes can't properly handle. It certainly doesn't appear to be a HW issue, at this point anyway. Will keep debugging.