@smathews
I often see this when the Missing Submission policy is set in the course first. Then, an assignment is created, updated, and/or copied into the course with a due date set to the past. Even if the due date is updated to the future, the Missing Submission policy would have already applied and doesn't automatically get removed.
A simple way to clear out the grades for an assignment affected by this is to do the following:
- Go to SpeedGrader in the course
- Hover on the assignment column heading and click the 3 vertical dots.
- Click "Set Default Grade" in the menu
- Leave the score blank and check the box to "Override already-entered grades"
- Click "Set Default Grade"
After completing this, the grades should be blank.