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

Another reason I think it’s an Apple Issue is the highlighted portion of the statement in the article

• It appears that Apple added this new setting, in part, to address complaints from Apple TV users who use Sonos speakers paired with AV receivers. Some users have indicated they hear popping noises or awkwardly low volume levels on certain channels, specifically with 5.1-channel content

What Sonos speakers “Paired” with AV receivers? How is that done? Must be some non-standard  Sonos integration with an AV receiver. The only way to bring an AV receiver into Sonos is via a Connect or Port...and that’’s NOT pairing 😂

Soundbars are also considered “AV Receivers”. Anyhow, Arc + Arc Ultra are anything but old.

• Continuous Audio: Select to turn Dolby MAT on or off. Apple TV 4K uses a continuous Dolby MAT connection for seamless transitions across audio formats without glitches or dropped signals.

When Continuous Audio Connection is on, older receivers might indicate an Atmos signal, but the original audio plays as intended, which may also affect the ability of a receiver to distinguish a stereo signal for upmixing if the receiver supports that.

 

Play audio in Dolby Atmos or surround sound on your Apple TV 4K - Apple Support

Based on that report, unless I was having an issue (which I’m not, on either Arc / TVs powered by Apple TVs), I’d leave that setting off. The only time I’d use it is if there was an issue. It is certainly off on my systems. 

I have to respectfully disagree with that statement. A soundbar isn’t an AV Receiver. Here’s a WiKi on both...

An audio/video receiver (AVR) or a stereo receiver is a consumer electronics component used in a home theater, home audio, or hi-fi system. Its purpose is to receive audio and video signals from a number of sources, and to process them and provide power amplifiers to drive loudspeakers, and/or route the video to displays such as a television, monitor or video projector. Inputs may come from a TV, FM, or AM tuner, satellite receiver, DVD players, Blu-ray Disc players, VCRs or video game consoles, among others. The AVR source selection and settings such as volume, are typically set by a remote controller.[1]

A soundbar, sound bar or media bar is a type of loudspeaker that projects audio from a wide enclosure. It is much wider than it is tall, partly for acoustic reasons, and partly so it can be mounted above or below a display device (e.g. above a computer monitor or under a home theater or television screen). In a soundbar, multiple speakers are placed in a single cabinet, which helps to create stereo sound and surround sound effect. A separate subwoofer is typically included with, or may be used to supplement, a soundbar.[1]

​@AjTrek1, Technically you are correct. However, I haven’t yet come across a TV manual that distinguishes between an AV receiver and a soundbar.

I suspect they’re ‘lazy’, and mean ‘any device that can accept and translate home theater signals’. In their world, that’s predominately A/V receivers, in our world, it’s both types of devices. 

Their world or mine. I’m not about to start referring to a soundbar as an AV Receiver. Especially not when it comes to Sonos. IMO there’s already too much crossover of terminology within Sonos. If Sonos starts referring to their soundbars as AV Receivers they’re unequivocally losing their minds.

Never tempt Sonos marketing types!

​@AjTrek1 

To be honest I think the 9to5MAC article is an example of bad facts research. It refers to some Reddit posts and the part you highlighted above imo really makes no sense. 
So at the moment I prefer to read what Apple officially says about the feature, what ATV users tell about their own serious investigations and at least what Sonos will tell us after they did their work/ analyses. 😉

Referring to Apples support article I think they are talking about „receivers“ and mean everything (av receivers, soundbars etc) an ATV can be connected to. So speaking of „older receivers“ imho could also include Sonos HC devices. The „older“ in this case may point to the Sonos software that might not be able to detect the correct format within mat signal if Atmos is turned on. 
I know… all suspicion, but just my 2 cents…

​@Schlumpf 

I hear you. However, IMO it’s just bad research on both sides. When lines are blurred between an AV Receiver and a Soundbar it only make for more mis-understanding by folks. I guess my having dealt with AV receivers long before Soundbars (especially Sonos) were a thing I’m a bit anal on the subject. When I say long before I mean when Heath Kits were popular (maybe that’s before your time as well 😂)