iOS 18.1.1 Auto-Lock interferes with timer alarm
If my phone is set to Auto-Lock in 5 minutes, a timer set for 5 minutes fails to sound. A 4:59 or 5:01 timer works fine. If you then change the Auto-Lock duration to, say, 30 seconds, then a 30 second timer fails to sound, but any other duration works fine.
You see the screen dim when there are 8-9 seconds left on the timer, then just as you'd expect the screen to turn off completely and/or the alarm to ring, instead the lock screen pops up at full brightness for 2 seconds and then the screen turns off - but no timer alarm.
Note that you have to set the phone down for this to occur, if you're holding it and watching, it won't autolock and the timer works fine.
I found an ancient post discussing the same bug from 2020 with no replies:
Auto-Lock and Timer - Apple Community
iOS 18.1.1
[Re-Titled by Moderator]
iPhone 14 Pro, iOS 18