Apple TV 4K: No Dolby Vision If Connecter to Denon AVR

The Sony is set to Enhanced.

If I understand the Sony documentation correctly, the TV would have such a setting for each HDMI port (with that capability) in use. If you connected the Apple TV to a different port than the receiver, that that would be independent for Enhanced setting. But if you have all that covered, then look elsewhere.

I'm using the ARC connection between TV and receiver.

Excellent. ✓

I've consulted the Denon manual and am using recommended settings.

The receiver in the chain seems to make the difference. To me it seems likely that a setting on there is making the difference, because the specs by itself are pretty awesome. You may want to get advise from Denon Support.

Your receiver or TV may need settings changed to allow for high bandwidth streams to/through specific HDMI circuitry. Sony has been know for the Enhanced setting for each designated HDMI ports. Not sure about Denon. See your manual(s).

I have the Denon AVR-S960H and Dolby Vision is working fine through it (Apple TV 4K->AVR-S960H->Sony XR-65X90J via eARC 3 input. Our menus are probably somewhat similar but your Denon is higher-end than mine. On my Denon the "4K/8K Signal Format” setting is set to ‘enhanced’ and as I said, Dolby Vision is working properly.

If you have confirmed that the 4K/8k Signal Format is set to Enhanced then the only other thing I can think of is checking to see if there’s a firmware update available from Denon.