Ok, this is is weird. Did some other research and found a thread suggesting that if you turn off HDMI controls/CEC in the receiver, the problem disappears. I did that and voila! DD and Atmos all fine. The Denon still reads “Multi Ch In + DD” now, which is a little strange, but that same thread said that’s because the AppleTV is doing the sound processing and passing that signal to the AVR rather than allowing the Denon to do it. If the source material is in Atmos, the Denon does say “Dolby Atmos.” I have no idea why any of this would be true but it seems to work