I tend to agree, the Marantz is a rebranded Denon or the other way around, but basically the same receiver as they are both the same company. I also have the A9G. I still get glitches on 4K on the Hopper and I have the fastest cables I could find coming in from Amazon in a couple of days. But other than it being the cable between the H3 and the Receiver I know it's can't be from the receiver to the TV, because just this afternoon I watched the first two episodes of The Expanse on Amazon Prime from my Fire TV Cube and it was perfect 4K HDR will full color depth gamut.
I've also read, which I will try if the new cable gives the same, that it seems the HDMI ports on the Denon's and the Marantz seem to be on a buss. On the Receiver forum someone was having the same problem and it was suggest to move the offending device closer to the Monitor 1 output.
As you know the Cable/Sat input is the farthest to the left while the output is furthest the the right. I have my FireTV hooked up to the Game or Media port (I forget which) and that is much closer to the right. So, if the new cable doesn't work, that's my next step, even if I have to rename and reassign all the inputs. If it fixes it, then it's the receiver. If not, then the finger is directly on Dish IMHO!!