I have a Samsung Galaxy S9, and my alarms on my Clock app have stopped working. This is ALSO happening to several friends and family, who all have an S8 or newer. Phones all have the latest app updates and OS updates installed.
Alarms on the Clock app just do not go off. They do not even have a "missed alarm" notification. They never go off at all. No one has any conflicting clock apps or alarm apps that would suppress these alarms from going off.
Do you happen to have your phone configured to restart every night? That will prevent the alarm from going off since you need to unlock your phone after the restart before it will start normal operation.