Audio passthrough, SONOS incompatibility

I’m running Apple TV 4K to LG C9 to Sonos. C9 has a passthrough, but ATV4K doesn’t. The problem that Sonos can’t get the formats straight if it’s not passthrough. Apple sends multichannel pcm that as I understand is not getting decoded properly by Sonos! They need Dolby Digital 5.1. Atmos content works ok, as Apple sends Atmos and Sonos receives Atmos. But for other formats it defaults to multichannel pcm! Why? Can anything be done with it?

I know there is way to put Dolby Digital 5.1 as default, but then it disables Atmos. Why? Can Apple switch between two formats or let us choose what we want? I’m about to replace Apple TV to something else that doesn’t have this problem.