Notifications

I have been having a similar problem, no notification tones for iMessages when phones are locked, for a few weeks for my iPhone 12 Pro Max and my wife's iPhone 8 Max, both running IOS 15.4.1. We get tones for SMS, but not iMessage. Message Notifications were always enabled. I tried several fixes found online to no avail. Since the problem was just with iMessage and not SMS, I decided to remove our cell numbers from iMessage and FaceTime and turned both off. We could then receive SMS from other iPhones (and Android phones) with a Notification tone. I then re-enabled our cell numbers/iMessage/FaceTime for our respective phones and now iMessage Notifications are working normally. I am not a programmer, but this appears to suggest a problem with iMessage/iCloud and not the iPhone or IOS, perhaps triggered by IOS updates. I hope this works for you.

Go to settings—-Notifications—- Messages— Allow notifications—Toggle ON and check all other options also ON