If you set Digital Sound Out to Auto, is there any difference?

You might have to manually select the Dolby Atmos audio track AFTER you begin streaming the Dolby Atmos content too.

I don’t have that TV, but I have noticed that the FireTV stick 4K and 4K Max behave this way for me too.

My TV does not support Atmos, so not expecting that, but default was stereo PCM.

I have to hard code the audio on the FireTV stick from “Best Available” to “Dolby Digital Plus” or “Dolby Digital” to force it to not use PCM.

on FireTV

Settings>Display and Audio>Audio>Surround Sound

try changing it from “Best Available” to “Dolby Digital Plus”  see if that gets you Atmos… they may even be an “Atmos” option as your TV supports it.

hope that helps.

personally, I have been disappointed with FireTV…. So many little quirks like this

Check that the HDMI input audio format on the TV is set to Bitstream and not PCM …and also that DTV audio is set to Auto (see screenshot).

Then It’s a case of sending DD+/(Atmos) or often ‘Auto’ audio output from your connected devices. 

Weird, while the fire stick audio settings imply it needs to be set as Best Available to get Atmos, it results in audio format of Stereo PCM showing in S2 app. When I changed this setting to Dolby Digital Plus, it shows in S2 app as Dolby Atmos  (Dolby Atmos DD+ though in About My System). So the issue was the fire stick, and it is preventing me from getting Atmos True HD

FireTV stick is really weird for me, thought it might just be  my TV.

I had a Beam Gen 1 and a FireTV Stick 4K (not max), I swapped the Beam for a Gen 2 and as I lost the remote for the FireTV 4K, I got a 4K Max as it was not much more expensive than buying a new remote when it was on offer.

The Max is the same as the 4K in that I had to force it away to get anything other than PCM. 
 

With the beam Gen 2 I had the option for DD+ so went with that.

TV apps all work fine, but BritBox on Max won’t load most of the time, Disney + will only play in HD and not UHD.

just tried an Atmos title in Prime and to my surprise It DID play ATMOS on the Gen2…. but not in Netflix or DIsney +
 

it’s a very buggy platform, thinking of getting an AppleTV to use instead.

EDIT…. Actually Apple TV will not do ATMOS over DD+, back to the drawing board.

Streaming services like Prime Video don’t use the lossless Dolby TrueHD version of Dolby Atmos. They use lossy Dolby Digital Plus instead because of limited bandwidth when streaming. Dolby TrueHD/Dolby Atmos audio is primarily used on Blu-ray discs.

So if i get Apple TV 4K, would it be True HD?

No all streaming services only stream in dd+

Not device dependant