I have using Nokia 6 last from 3 months and have done factory reset and soft reset 3 time to rid alarm missing issue. I have update latest security patch and operating system Oreo 8.1. have to cleared cache every week. But alarm problem is same.
Did you try factory reset and set up the phone fresh, without restoring a backup? Let the phone update all built-in apps, and confirm that the alarms are stable, before adding your usual apps, and before customising any Android settings. (Alarms and notifications has always been reliable on my Nokia 6 and 8 FWIW).