Ah okay, my receiver is an old Denon 1613. I checked on their specs page but it didn't mention if DD+ ARC is supported. It only says it supports DD+ decoding.You've just said "Old Denon" a few times without saying which model is. But not many TVs support DD+ over ARC, and I suspect it's probably likely that your Denon doesn't support DD+ over ARC. If that's the case though, it will tell the TV using the EDID handshake, and the TV won't even try to send it. It'll send plain DD over ARC instead.
You can check on the front panel of the AVR / the Android App (if it has one) / the built-in Denon webserver / what it is actually receiving from the TV (as it's not easy to use the Denon's menus when on a TV source).
The front panel shows DD+ but the sound is distorted somehow.