I’m having the same issue.
iOS is up-to-date.
Restarted the phone.
Done a complete wipe out reset and synced everything back from iCloud.
Reset network settings.
I have turned off notifications on my other devices. And third party apps.
I have hide alerts toggled on and the bell from the message center when you slide it, I turn it to mute. Sometimes mute seems to un toggle itself on its own.
I have hidden alerts from each individual in the group, separately. There are no people in the group that is not in my contacts.
Synced messages in iCloud.
Toss in left the group chat and started a new one.
my 15 pro max is also brand new because I was having issues with my Bluetooth as well, so Apple replaced my phone after warranty because the issue started pre-warranty ending. (This is my second replacement from apple because of this issue with Bluetooth)btw day 2 with the new iPhone and BT is still an issue.
all of this, and I am still receiving alerts.