I have the same problem and my conclusion is that it's the macOS Calendar that syncs wrong data to iCloud, when adding daily events in an iCloud shared calendar (the calendar belongs to my wife's iCloud, shared to my iCloud account)
- macOS Calendar shows correctly created events, no alarms
- My wife's iCloud sync to iPhone/iPad shows correct events, no alarms
- My own iCloud sync to iPhone/iPad shows the events WITH alarms...
- My own iCloud web shows the events WITH alarms
It used to be fixable by removing iCloud calendar from iPhone/iPad, but today it consistently syncs the events back WITH alarms - on both devices
Ignoring the Calendar settings for default alarms completely (but only on my iCloud account - not my wife's/the owner of the iCloud calendar)
It appears macOS Calendar syncs shared daily events WITH unconfigured alarms, ignoring the macOS Calendar alarm settings (no alarms on daily events) - this was a problem on macOS 10.13 and 10.14.