I have noticed that all the latest series of Amazon Prime shows are missing multi-channel (5.1) sound on apple TV 4K (and I suspect apple tv 4th gen).
I have an HD TV and an AV amp which is not Dolby Atmos compatible.
The follow series are missing 5.1 sound on my set-up:
Jack Ryan - series 1
Bosch - series 4
Goliath - series 2
All the series were first made available this year (2018)
The older Amazon series of these shows (and both series of Man in the High Castle which dates back to Dec 2016) do have multi-channel 5.1 sound.
I get 5.1 sound for all of the recent Amazon prime shows if I use my old Amazon fire tv streaming box so it's not a problem with my AV amp.
My guess is that all the latest series of these shows have dolby atmos sound but that Amazon have not chosen to make this available to apple tv app for now so that it is an exclusive feature of the new amazon fire TV.
I'd be interested if others can confirm my findings about these Amazon prime shows.
Other non-Amazon series and films seem to play fine with multichannel sound.