I am in the process of downloading iOS 15.1 Public Beta 2 and fingers crossed it fixes the Apple Watch bug and hopefully this bug. I am increasingly thinking the iMessage notification problem we are all experiencing is some sort of bug with the Focus feature that has been added in iOS 15. Although I have not been able to restore the iMessage functioning as before, I did fix a problem with an app I must use for work called Haiku, which is the mobile phone client for the Epic patient health record. For the first couple of days with my new iPhone 13 Pro, I would not get any patient or colleague notifications in Haiku until I made sure it was not involved in any Focus scenes etc. My guess, Apple engineers will determine that is why iMessage has stopped working as expected for us.