I saw the same results with my FireTV 4K sticks and Disney+. Dolby Vision with 5.1 DD+.
Right now the best devices I have for Disney+ playback seem to be my Rokus and Apple TVs. Since I can get Atmos from both of them. And HDR10 from the Rokus and Dolby Vision from the ATV 4Ks.
I hope they enable Atmos from the FireTv stick. Because I would prefer to use that over the ATV 4K. That way I could get DV and Atmos.
Don't understand why you prefer the stick since AppleTV 4K seems to support just about everything, both audio and video according to the Apple site.
I'm trying to make up my mind whether to spend 3x the amount for the AppleTV or just buy the 4K Stick...