When you’re playing content on the TV, what does the Sonos controller’s ’now playing’ screen say the Sonos is receiving?  What are you watching on the TV when you’re testing?

I have the same setup myself, but with a playbar..
Had the same problem or so I thought.

Actually feel a bit stupid but here is my discovery.

Got a new TV, Samsung 55" S90C 4K OLED Smart TV.
Everything worked perfectly before but on the new one no Dolby 5.1.
Watch all TV through Apple TV 4k. When I watched regular TV or movies, only my playbar produced sound.
The first step is to see if everything is set up correctly in your App.
Settings, system, system settings, about my system, Sonos system info.. There you can see if all devices are updated and connected correctly.

Well, back to my discovery when I watched TV there was only sound in my playbar, after a lot of googling settings on SONOS, Samsung and my Apple TV I discovered that it was a simple setting on the Apple TV under sound that did everything was broadcast in stereo, so clearly no sound comes out of one's surround sound. Switched to default and now it itself switches to Dolby 5.1 when this sound format is broadcast.
You must be sure that what you see is recorded in surround/Atmos, otherwise only sound will come out of your Arc.

p.s
Remember that your Samsung must also be set correctly in sound settings.

Moderator edit: removed pictures with serial numbers.