This issue occurs when students cannot see an assignment after it has been undeleted by the teacher.
All affected grades will have to be republished in order to update the progress report and make the affected assignment visible again after undeleting. On the right side of the main Gradebook spreadsheet:
- Click Progress Report Publishing.
- Publish the grades again as you usually would.
If you have performed the steps above, students should be able to the undeleted assignment.