Same problem here iPhone 12Max problem started with install of IOS15 final release. 15.01 same issue. Home is a Shared home w 130+ devices connected to the Home. Works OK on iPad for same home and UserID

The No response issue is likely caused by a stuck hub. I always search for what hub is connected an unplug it. The system returns to normal as soon as it reconnects to a new hub. It’s always a HomePod that gets hung up and causes a no response condition.