iPhone 12 Pro Max - no alert for texts when I'm on the home screen or phone is locked

I've had the iPhone 12 for a while and yesterday, all of a sudden, the text alert sound stopped happening when I'm on the home screen or phone is locked. It works fine if I'm in the actual text screen itself. Before anyone asks I've checked everything - it's not on silent, volume is up, checked notifications settings, sounds & haptics, do not disturb etc. All are set correctly and I've even gone through them comparing with my work iPhone and there is nothing different. I've had someone else check as well and they can't find anything. This thread seemed to have stopped a couple of years ago saying a fix would be provided by Apple Developers but not sure if it's a rebreak in their code. Has anyone else started to get this problem recently?