Does it work as expected with the Dune connected direct to the TV (removing the AVR)?
In that AVR settings there is a switch for 'enhanced mode' support on the HDMI ports. This should be enabled:
http://manuals.denon.com/AVRX2400H/EU/EN/DRDZSYzvebvotm.php
The AVR configuration I have checked and it was already in the "improved HDMI" setting as I mentioned before, I have the other M 9702 player and it plays both HDR and Dolby Vision without any problem, it only makes a black screen the first time, which is when It does the automatic refresh change and that's it, it doesn't matter whether the movie advances or the chapter passes, the image is always on the screen without any problem.
I have connected the Dune directly to the Oled LG B7 and it does exactly the same, 5 second screenshots every time I go forward, backward or change chapter.
You will tell me what else it can be, because I have it with all the settings that you indicate and the problems are not solved.
I also have problems with HD audios, they are always out of sync for about a second with the image. The other audios without problems, but the HD always out of sync with the image.