Uninstall and reinstall. The titles will still show 5.1 but start one playing that you know has atmos. Iron fist and lost in space do. After than you'll get the atmos tag on everything that has atmos. I did this yesterday and it worked.
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You will see atmos from the ATV. It can't decode atmos and must pass the metadata so your receiver will still show atmos.
Thank you, you are correct. Just verified right now, my Denon shows Atmos when playing Jurassic World