Is there a difference in sound when the file is in Dolby Digital Plus 7.1 vs. DD+ 5.1 ? vs. TrueHD 7.1?
Unfortunately the AppleTV only supports atmos when it's in a Dolby Digital Plus 5.1 container. DD+ 7.1 or TrueHD is not supported and will be converted to multichannel PCM.
https://support.firecore.com/hc/en-us/articles/217735707-Audio-Options-tvOS-