mod_checklist: Activity due date not added to Calendar and Upcoming events

The activity due date is not added to Calendar and Upcoming events for mod_scheduler activity

Environment:
Moodle - 3.9.3+ (Build: 20201217)
Checklist - 2020111400

Replication:

1. Create a course.
2. Turn editing on.
3. Add a Upcoming events block.
4. Add a Checklist activity.
   • Name: checklist_aaa
   • Activity Completion
     • Completion tracking: Students can manually mark the activity as completed
     • Expect completed on: [set future date, say 3 days from now] and tick Enable.
5. Click "Save and return to course" button.
6. Back on the course contents page, you will see that the "Upcoming events" block does not have any entry at all. Click the "refresh" button on your browser. There is still nothing displayed in the "Upcoming events" block.
7. Click the "Go to calendar..." link
   Result: There is still nothing displayed. No entry is added onto the calendar