ATV 4k remote constantly disconnecting

Check the ATV for updates.

And, reboot the TV and the ATV by unplugging them from the wall for two minutes. Check both of them for updates.

Sonos receives the ARC signal from the TV. If the TV stops sending data, there isn’t much that Sonos can do.

If it continues, I would recommend that you submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it. Don’t post the resulting diagnostic number here, their lawyers get sensitive about GDPR.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the Support staff, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.

I might have read somewhere the ATV remote needs updates too.

Do you command the Arc with the ATV remote directly or through the TV? This last option might be more stable.

As far as I’m aware (and I use two ATVs), the remote updates with the ATV…unless it’s broken. There is a way to reset it. 

As written, you should make sure all devices are updated, SONOS, ATV and the ATV remote.
The ATV's firmware/software is now on tvOS 26.2 and the remote is on 0x33 (should be the latest but I'll check). The remote only updates itself if it is close to the ATV and charging or has over 50-60% battery and the ATV must be off/in sleep.

If this trick doesn't work ( How to fix Apple 4K TV remote not controlling Sonos soundbar ) 

I would then remove the remote from the ATV and reinstall it.

Remove a remote from the Apple TV
Go to Settings on your Apple TV.
Select Remotes & Devices.
Select the Remote & Devices app.
Select the device you want to remove and tap Forget This Device.

Pairing: Hold down the Back (or Menu) and Volume Up buttons for 5 seconds, and place the remote on top of the Apple TV to complete.

Yeah, Everything is up to date, and I’ve already tried unplugging, etc.

And, in my original note, the above is exactly what I need to do 2-10 times every time I turn on the TV. Eventually, volume control works again. But it is getting tedious to do the above so many times every time I sit down to watch TV.

Have you checked your settings on ATV under remote control?
What settings do you have, could something have been changed by mistake or update.

To my ears it sounds like something is wrong with the connection to your ATV.
The ATV's remote control can control SONOS but it is only used as such when your ATV is turned off and you are listening to music, otherwise the ATV's remote control controls sounds from the ATV which are then passed on to the TV which then controls the SONOS soundbar via HDMI.

Are you sure all HDMI are correctly placed in the various ports? Sonos eARC.
And the settings on the TV are correct.