Seems odd, certainly. Grouped rooms should be able to play the same content when grouped, despite the 75ms delay between the Arc Ultra’s room on the TV input, and any grouped rooms.

This suggests a few things to look at. First, make sure all ‘rooms’ are on the same firmware by checking for updates in the Sonos controller. I’m assuming all ‘rooms’ are using S2, since you’re talking about an Arc Ultra. Next, check for wifi interference , the simple fact is the bandwidth necessary for a Sonos line in is more than the average music stream, it might be affected by too much interference to allow connection between the router and the rooms. 

But if neither of those solutions offer relief, 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, they 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 seem to recall another recent thread where there was an issue with tv audio to grouped rooms. It may be worth mentioning that when you speak to support.