Duplicated activity's due date does not display in calendar

When an activity with a configured due date is duplicated, the duplicate activity does not display in the calendar. Additionally, if activity's title is changed via edit title, the activity continues to not display. User must save the activity via setting page for the activity to display in calendar with due date.

Steps to Reproduce

1. Access a course as teacher or admin.
2. Create an activity with a due date for near future.
3. Duplicate the activity.
4. View calendar.
   Expected - Duplicated activity displays with due date.
   Actual - Duplicated activity does not display with due date.