Strange it would do that , are you on android 11? Can you reboot the phone?
I'll leave a Link to register so you can communicate here as a guest account you can only post questions but can't reply https://forums.androidcentral.com/showthread.php?t=409154
Welcome to Android Central! Is this when the alarm sounds while the phone is idle? Are you saying that after dismissing the alarm, the screen goes back to sleep? Are you able to wake the screen again without difficulty?