Try unplugging the Arc and Boost from power for a couple of minutes and reboot your router.

Did power unplugs and router reboot and same issue after testing again.

Describe your setup. What model TV do you own? What is your Atmos source? From what device(s)?

Samsung - 65" Class Q70T Series QLED 4K UHD Smart Tizen TV, Apple TV 4K, Arc/Sub/2 One SL’s. Sonos Boost.

Have you always experienced this issue with Atmos audio? Do you have this problem with the TV’s internal apps or just the Apple TV?

Be sure you have the latest firmware installed on your TV.

You may also try disconnecting everything from the TV and unplugging the Arc and TV from power for about a minute. Power the TV and Arc on again but keep the Arc and Apple TV disconnected. Make sure Anynet+ and eARC is enabled on the TV and redo the TV Setup process in the Sonos app. Connect the Arc to the TV when the app instructs you to. When the Arc is connected properly, reconnect the Apple TV.

I had been through the setup again when I was working through Sonos Chat. Could any of this be a network issue. I am thinking about running Ethernet to all the Sonos speakers.

More than likely it’s a Samsung/Sonos issue. Samsung TV’s with eARC have a history of problems with the Arc.

Do you experience the same problem from the TV’s native apps or just the Apple TV?

You can try wiring one of your speakers (not a surround or Sub) to the router to improve your speakers’ network performance but I’m not sure it will make a difference.

You might also consider using a different HDMI cable for the Apple TV. Are you using the Sonos-supplied cable between the Arc and TV?

TV’s native apps do not seem to play Dolby Atmos when available. I have swapped hdmi cables already to test. I am using hdmi 2.1 cables and the Sonos hdmi that came with the arc. Samsung does not show any updates available through the settings.

What is Digital Output Audio currently set to on your TV? Does either the 'Auto' or 'Passthrough’ setting allow you to get Atmos audio from the native apps or make a difference with your audio drops with the Apple TV?

I have tried both Auto and Pass Through, issue persists on either. Does not seem to affect native apps as far as getting not getting Atmos from Smart Apps on the Samsung.