I did some testing to see how this thing works, as I don't usually pay it much attention (it just does what it does):
I created a calendar event, set an alarm for it. Made sure it synced to both the iPhone and iPad. Turned VPN off on both.
Notification was triggered on all three devices. I dismissed it on the Apple Watch. It vanished from the iPhone's lock screen as it should.
I kept turning the iPad's display on to see the lock screen, and over the course of some fifteen minutes the lock screen notification did not budge. As soon as I authenticated and locked the device again, the notification was gone from the lock screen. Didn't even have to open my calendar app.
This seems to match with my normal experience. If the iPad is in active use (that is, I'm unlocking and using it), calendar notifications don't linger on the lock screen.