Turn off Auto restart on your phone. If you search for "Auto Restart" in your settings search window it will take you to the option. Turn this option off and your alarms should start working.
This is an Issue with the Auto Restart option. It kills all your alarms after it reboots.
Go to Settings > Device Care > 3 Dot Menu > Auto Restart and turn it off ...if its on.
I have exactly the same problem. I have done all the suggestions and more. Nothing has worked. Annoyingly, this seems to have happened with my S8 after an update and then transferred to my S10. If I set my alarm for 10 minutes time it will sound. It must be a deep sleep issue. I even downloaded an external alarm app and that didn't work!! Trouble is, if you do a total reboot aren't your settings saved on Google cloud therefore you will have the same problem when restarting?
Did you guys ever resolve this issue? A current Note 10+ owner has the exact same issue and I've checked all her settings. Auto restart is OFF. Issue still persists when she wakes up and her phone is off thereby missing her alarm appointments.