What is your source of what you’re testing? Have you checked your audio settings on the Apple TV? 

For instance, most content on YouTube is stereo only, but your Apple TV audio settings may be up-rezzing it to Dolby 5.1

It is a regular local app to watch commercialized channels, 5.1 has worked previously so it is strange that it doesnt now. It says 5.1 on all channels now in the Sonos app, previously it was a mix where 5.1 was on 10% of the channels.

Not sure why no sound comes from rear speakers at all, I mean both Stereo and 5.1 should generate sound from rear speakers, right? 

I checked all the settings and everything is set to Auto for sound output (i could choose between 5.1 and Stereo) both on tv and Apple TV. When I play Netflix on Apple TV then I get 5.1 sound, but not on the commercialized app.

I used to have football games broadcast in stereo come in on a 5.1 signal. And it was a frequent thing for a station to send news programs only in stereo, while the station was broadcasting in a Dolby Digital signal. There’s no guaranty that the show you’re watching is actually in 5.1, no matter what the Sonos says it’s receiving.

Your best bet is always to test on a movie channel, with a recent (last 10 years or so, newer is better) movie. 

No, stereo broadcasts, even when in a 5.1 Dolby Digital codec, wouldn’t have any information carried on the surround channels, they’d be by definition stereo, with just front right and front left signals encoded in the codec, and nothing in the other channels. 

My guess would be twofold. First, there may be settings for audio for this “commercialized app” that need to be adjusted, or second, the show you’re using to test with just doesn’t have a full 5.1 soundtrack, but is still being broadcast using a 5.1 codec. 

If Sonos is working with Netflix, then Sonos is working correctly. If you’re having trouble with this app, then you’d probably want to speak to the folks who support that app.