I have noticed that both incoming phone calls and alarms with iOS 12 (and iPhone Xs) ring loudly until I pick up the phone. I don't know for a fact but suspect this is an intentional design change. It works very well for me as I have the ringer volume set all the way up so I can hear it if in a loud environment but it doesn't need to be (and is annoying to be) that loud when the phone senses that I have heard it. The alarm (and incoming calls) on mine will both continue ringing at full volume if I don't pick up the phone.