Apple Home

Did you update to the new Home app architecture on your iPhone or Mac Pro?

If you did, that likely cut your iMac running Big Sur out of access to your devices.

Upgrade to the new Home architecture - Apple Support

M1 Mac mini, macOS 13.3.1;  Watch, series 7 GPS/cellular, watchOS 9.4; iPad Pro 2nd Gen, iPadOS 16.4.1; iPhone 12 Pro, iOS 16.4.1; TV 4th Gen, tvOS 16.4.1; HomePods, audioOS 16.4.1 and Airport Express w/ Airplay 2

It's included with Big Sur so I would hope so. 😎

I see. Personally I don't use Home on any version of macOS so I can't really help. Hopefully someone will.

See if this helps: If your HomeKit or Matter accessory isn't responding in the Home app - Apple Support

Since I don't use Home I don't have any suggestions but hopefully someone will see this post and have some idea.

FWIW, I put out a feeler to see if someone has any ideas.