Hey all. I have a Denon 3700x with an Apple TV as my primary streaming device, with a 5.2.4 speaker setup. Recently, somewhat out of the blue, various streaming services are no longer (apparently) sending Atmos through the way they used to. Disney+ and Apple TV+ still do; the Atmos text comes up on the Denon screen and it is very clearly Atmos when you listen to it.
But HBO Max, Prime, and Paramount+ are not seeming to send Atmos, all of a sudden. HBO Max, in particular, was working fine until the last few days (Prime I hadn't used in a few weeks so it is possible it was recent as well, Paramount+ I just started using, so again, nothing to recently compare it to). The exact same media no longer comes up in Atmos. It defaults to Multi Channel In, and I can't select Atmos no matter how hard I try. I can get sound out of the height channels by switching to Multi Ch In + Neural X, so it isn't a speaker problem, per se.
To bypass the Apple TV itself, I used the default TV apps (LG) and signed into HBO Max there, and it is not clear to me whether it is picking up Atmos or not. The receiver says Multi Ch In + DD (so, not Atmos), but when I cycle through the receiver options, I come across an option for Atmos/DD Surround, and when I select that, it then shows... Multi Ch In + DD. It does come through the height channels on that setting, but even Multi Ch In + Neural X will send output to the height channels, so I don't know if that really is Atmos or just mimicking it.
I have done some searching around to figure out what the problem might be, but nothing seems to answer it. Not clear whether it is the Apple TV, the receiver, the Apps themselves (though that seems unlikely because this has all happened kind of at the same time). Nothing I have found explains it. The Apps are updated, my download speed (at least according to speed tests) is fine (300 mb range), and of course, Disney+ and Apple TV+ are sending Atmos fine, so there's that, too.
I have never entirely understood what Multi Channel In really even is, so it is possible I am just entirely misunderstanding what is going on here, but at the most basic level, when I watched certain movies a week ago on HBO Max, they were showing Atmos on the receiver and clearly sending the surround/height sounds, and now they are not. Any advice/help/suggestions would be much appreciated!