Audio not playing on AppleTV app (but everywhere else)

[blubleublue]

Hi! 

I have two fresh installations of Jellyfin 10.10.3 on

A) Proxmox mini PC within a LXC (mainly for testing)
B) Unraid NAS within a docker container (will be the final "production" installation)

Both systems have very similar hardware: Intel N100 + 8GB RAM
Jellyfin on both systems mount the same dataset/volume to build their library from the NAS. So they access basically the same data.

The problem on both installations:

One particular movie with 7.1 passthru audio is not playing sound on my TV when using the AppleTV app. It works fine on the same TV with the AndroidTV app version of the TV OS.
I noticed in the logs that it is direct play, and not transcoded.

I also tested the same movie converted to AAC and then audio works on AppleTV, too.

Can someone help me troubleshoot? Is this a transcoding problem? Or a problem with the AppleTV client?
What more info do you need? Logs? If so, which ones?  I fiddled with the transcoding settings now for hours and do not know what to do next...

I am about to digitize a ton of my physical movies and would love to avoid having them all in AAC but rather the RAW audio for my (future) home cinema audio system.
Knowing that I can keep the original audio and Jellyfin transcodes according to device/client`s needs would give me peace of mind.

Thanks in advance