I am having this same issue (Galaxy s8). My home screen click widget is telling me I have an alarm set for 6:43pm but it is nowhere to be found in the Clock app. I've gone into the Clock app, first turned off any/all alarms with no change. I've even deleted all the alarms and world clocks, etc. No change.
Went into safe mode to see if any now-disabled apps were causing it but the alarm is still present. I've gone down the list of available apps within safe mode and nothing regarding alarms, reminders, etc..
Is there a way to find out specifically what apps use alarms/reminders? Trivial, maybe but it's annoying nonetheless.. Help!