Sorry, I’m confused about your issue. Your Apple TV connects to your TV set, your TV set provides an ARC signal to the Sonos Arc, which plays it. This implies that both the Apple TV and the TV set need to be turned on. Or, potentially, you might send a stereo signal via AirPlay 2, but the Apple TV would still need to be turned on, if it’s generating a signal to send to the Arc. And I’m not sure how you would be able to see what was going on with the Apple TV without turning on the TV set too. 

What does this have to do with HomeKit automations? Isn’t that an issue with Apple, and not Sonos?

Hello, the last year worked perfectly for me. the Apple TV doesn't need to be on to function as the brain of the house, and the Sonos bar can be added as a speaker to HomeKit. the television was not necessary until last year. It even worked with the Apple TV LED off. I think the signal is sent via AirPlay

Then you may want to contact Apple, as it seems as though something has changed in HomeKit. Sonos really isn’t involved here, it is merely a receiver of data sent by Apple.