ATV 4K has been updated and now everything plays in Atmos.

Have you read the tooltip?

’Apple TV uses a Dolby MAT connection for glitch-free playback across formats. Older receivers may indicate an Atmos connection, but original mixes will not be modified.’

With the new Continuous Audio Connection, using Dolby MAT on everything, Sonos will register everything as Atmos, it’s not playing Atmos on everything though, only on Atmos content. 5.1 will play as 5.1, it just has a Atmos badge at this moment. 

I’m sure there will be an update where Sonos can differentiate the Dolby MAT having Atmos metadata or not.

And for now, Sonos still doesn’t support/chase beta releases, I suspect to many hours chasing things that may or may not actually release. If Sonos were to make any changes, I’d expect it to be after Apple releases 26.4 as a final release, and not before. 

I have to say it…

I’m glad Sonos makes its beta testers sign NDA’s. IMO it’s articles like the one sited by the OP and then posted in a community like this that do more harm than good.

I’m an Apple “fanboy” even in my advanced years. Some might say I live and breathe Apple; and also Sonos 😂. However, that’s my business...each to their own.

However, I hate that Apple software beta’s; unlike Sonos, always seem to make it into the wild before it’s time (admittedly with the exception of the Sonos App debacle in 2024). Frankly, this ATV update regarding Dolby Atmos and popping issues in soundbars apparently has nothing to do with Sonos and never has.

That said….is enough.

In the past Apple has had a habit of making late changes just before releasing an update. 3rd parties working with the beta’s got blindsided by the formal release. Of course the in-house Apple applications were updated and worked. This causes Apple users accuse the 3rd parties as being incompetent and vow to avoid 3rd party applications.

I am not an Apple “fanboy”, but I have an iPad that I must use for work applications.

From what I’ve seen, when there is any issues between integrations with Apple/Googe/Amazon and smaller company’s hardware or software, the default assumption is that the smaller company is to blame.  It doesn’t really matter what sort of communication and timing occurred.

If only Sonos had followed that philosophy when they released the new app…

So now after TVOS 26.4 was released, I hope Sonos will start to analyse the situation and try to adjust their audio format recognition. Imo there has to be some kind of steady check for format changes during the continuous mat stream. 
At the moment I prefer to deactivate the option, because everything is shown as Atmos. 

I’m not sure this a Sonos issue. In the AppleTV settings app if Dolby Atmos is turned off the Sonos App shows the audio stream codec correctly.  Therefore, IMO the ATV app is sending false audio information to Sonos when Dolby Atmos is on. Meaning Dolby Atmos identification within the ATV is hi-jacking the audio identification for lessor codecs (if that makes sense 🤔).

But what do I know….🤔🤔🤔

After further investigation I found that turning off “Continuous Audio Connection” (CAC) in ATV resolves the issue of every thing showing as Dolby Atmos. So my suggestion is to leave CAC turned off; unless there is need for it.

Yes, that (to turn off CAC) is my final decision at the moment and keeps the status like it was before. 

I‘m also not sure, if it depends on Sonos or Apple to fix the „always Atmos shown“ issue. I was guessing Sonos, because Apple says that the issue could appear on older AV receivers and not in general. 
I suppose that Sonos misinterprets the mat signal as „definitely Atmos“. If Atmos support is turned off on ATV Sonos gets no mat signal that could be misinterpreted. Maybe if Sonos gets mat there has to be a double check if it’s Atmos inside mat or another format. 
But also maybe you are right and Apple has to add the exact format information within the mat signal. 
ATV User experiences with other non Sonos Atmos receivers would be interesting to see how it works there.