Had this before, now also with the new "Reminder Daemon". After a reboot and fresh login, I get reminder notifications for an event 2 days ago. The event is a recurring event, every week on tuesdays, but the last instance was moved from tuesday to wednesday. Now (friday) I get the reminder of the wednesday event and I can not dismiss it (the notification comes again and again).
The "can not dismiss" issue was the same as in bug #453567 which is fixed now. Somehow in ~/.config/kalendaracrc I had some old Suspended events - these are retriggered on start of the reminder daemon. Since the "Dismiss" action was not working, I could not get rid of them. Let's close for now and see if it happens again.