I wonder if its some setting in the AVR, or an ARC setting on the tv. When I get a chance I will try it.I only use apps Netflix, Amazon Prime and You Tube. Right now I'm trying to see The Midnight Sky on the tvs Netflix and I can select Dolby Digital Plus but there is no sound from the receiver. If I select regular DD I get sound. What is wrong? You can see in the second image that Netflix says it has Atmos and the tv lets me sellect DD+ but I get no audio from the receiver.
Do you have the denon app on any devices? When I use the tv apps ond not my firestick, I have to use the app to adjust audio and stuff as there is no gui when watching for 4k. You might also want to post this in the appropriate Denon forum for your 6400, I have the 3400 so it should be very similiar.
Currently watching football but will check later and post the results for you.