05.10.15 works for me.: Apple TV 4K (Second Edition) >> Denon X3300 >> LG C9. DV works again - no flashing, black screens, etc.
You may want to check for a Denon firmware update. A day or two after this problem occurred I received an OTA FW upgrade for my Denon. There is a post in the AVS Forum that some AVR's may need a FW upgrade too.
Mine worked immediately after I updated the TV.