Alarm only rings when phone is unlocked

Starting a few days ago, the alarm on my iPhone will only go off if the phone is unlocked. If it is unlocked and I am actively using my phone, it goes off normally. However if the phone is locked (such as if I'm asleep) the alarm will not ring until I press the home button to wake the phone. This obviously causes huge problems and somewhat defeats the purpose of the alarm.

Details:

I'm on iOS 11, but this happened a while after I got 11, so I don't think it's necessarily tied to that. I received the update for iOS 11.1.1 today and am installing it now to see if that helps.

Phone is iPhone 6.

This has occurred with multiple alarms that I have set, and has consistently occurred for every alarm since it began a few days ago.