Amazon Prime on Apple TV 3rd gen - no 5.1 audio

Hi all - there have been a few topics on this question, but none have been resolved as far as I can see.

Has anyone been able to get 5.1 audio on Amazon Prime Video content, on the Apple TV 3rd generation?

I can get 5.1 audio on Netflix and Apple TV+ content, but just not Amazon Prime. And the Amazon Prime Video Help page states that my device should get 5.1 audio:

https://www.primevideo.com/help/ref=atv_hp_nd_srchr?nodeId=GTAQMQBSRQKKE9FJ

And finally, as I'm contemplating upgrading to the 2021 Apple TV 4K in a few months - anyone able to confirm that there are no such audio output issues on that latest device?

Thanks for any input.