This is not the ATV 4K nor is it related to cables, etc.
The issue is with the LG C9 firmware. Around the time your problem started LG updated the firmware to version 05.10.04. Any DV capable device connected through an AVR, such as your Marantz, and then to the LG experiences the problem. I have the same exact issue.
LG is well aware of the problem and is testing/perhaps rolling out a fix already. It MAY be 05.10.15. Best bet is to wait for the update and use HDR as you are doing. I'm doing the same.
Here is a thread that has information in a number of posts. The user "HiFi4Vision" is credible.
https://www.avsforum.com/threads/official-2019-lg-c9-consolidated-info-faq-troubleshooting-firmware-more.3075344/page-80
Suggest you start with post #1583 and read thru.
This is the LG link (assuming you are in the US) where you can see what the current firmware is. You might want to consider turning off automatic updates. I wish I has not turned mine back on.
https://www.lg.com/us/support/product/lg-OLED65C9PUA.AUS
There is also discussion that your AVR might require an update. Oddly enough my Denon got an update a day or two after the initial occurrence. They didn't say what it was for. You may want to check for AVR updates.
Hope this helps. It's complex and difficult to tell what is creating the issue. I was lucky - the LG wanted to do an update - I let it - and immediately after I played a show that was in DV and got the blanking/blacking.
