Is there a way you can turn off Atmos on your receiver? I know on my Denon, I can turn Atmos on or off, and when I turn it off, it displays DD+ when watching Avengers: Infinity War, Ready Player One, or The Equalizer.
Careful, with the rebranding going on recently, Amazon Prime Video is both a subscription service (titles with the Prime flag) and titles you actually own (either purchased on Amazon Prime Video or redeemed thru Movies Anywhere).
I have yet to see a recent release on Vudu list 7.1 audio. At best, Vudu streams in Dolby Digital + for most titles, and their Atmos is an extension of DD+, not lossless Dolby TrueHD like we get on Blu-ray and UHD Blu-ray.