Late notification of calendar entry

My calendar event takes place at 15:00. I set it up to be reminded 10 minutes before the event time.
The notification pops up at 15:23… what happened?

Or should i use the Tasks app for events i want to be reminded of? But this implies double entry. Calendar+Tasks.
I never used the pre-installed app Tasks. Instead I’ve always used Calendar and Notes which do the job for me, apart from this reminder issue.