Hi Ferdinandspirit,
Thanks for posting in Apple Support Communities. It sounds like this happens with certain events and that you have checked notification settings. Right?
In that case, are these shared calendar events? It could be that you are getting a notification due to someone else modifying the upcoming event. Try these steps from Share iCloud calendars on iPhone - Apple Support
Turn off notifications for shared calendars
When someone modifies a calendar you’re sharing, you’re notified of the change. You can turn off notifications, if you don’t want to receive them.
1. Go to Settings > Notifications > Calendar > Customize Notifications.
2. Turn off Shared Calendar Changes.
If the same continues, are these events from a particular calendar? That might narrow things down.
All the best.