What causes Time-Sensitive notifications on iPad?

Since upgrading my iPad Gen 9 from iPadOS 17.7 to 18.0 (then further updates) I've had a 'time-sensitive event' notification every day at 8pm. Touching the notification just dismisses it.

I've looked in Calendar and Reminders but cannot find the event that is triggering this daily notification.

How do I find it?

[Re-Titled by Moderator]