alarm loud when setting but not when ringing

Not so hidden or new, but is a feature which came out with iPhone X, when FaceID was first introduced. To me, it's one of the better features of iPhone. You can turn it off if you like. But the virtue is that the phone reduces the sounds of incoming calls and alarms when your phone knows you are looking at it. In an office setting, it's a WONDERFUL thing. The logic being that you are aware of what's happening, so why does it need to be loud?

If you want to turn off Attention Aware, go to Settings > Accessibility > FaceID and Attention > Attention Aware Features

Are you looking at the phone when the alarm goes off? If you are, your phone likely has attention aware enabled, which reduces the sounds of alarms and ring tones if you are looking at the screen.

have you ever tried using SERI to set alarm ?

Wouldn't have bothered me if you had!