iOS Alarm volume decreased after being initially loud - fixed!
This issue is because the dimming of alerts is linked to Face ID activation. When you look at it to see the time, it dims the alert. To turn this off, go to:
Accessibility > Face ID & Attention > Aware Features
Turn off as per below:
I found another thread via Google but it was locked, so have raised this one. Hope it helps.
iPhone 13 Pro