You asked in the original post if Dish down mixes it. They don't do a thing except pass along what they have received. I don't think they have the workforce to individually set the audio criteria for every bit of programming they air.
I'm sure you are correct. Found something else today of interest.
I watched Jack Reacher on my new Roku Ultra with the Prime Video App. The audio was in 2 channel stereo PCM. This is a 5.1 movie.
Curious so I played it with my Sony BluRay player using the Prime Video App. The audio was correct! DD 3/2/.1
So now I am curious if Dish, Roku , etc. are just passing along or actually reprocessing (perhaps in the Hopper or box) the audio before delivery to the AVR.
Very strange .. I could not find a Prime Video App for the hopper to check.