If the Assignment was not added by the employee this won't have gone through an approval rule so the status will show as blank.
If you are filtering on Assignment status for Equals Approved, this would need to be amended to include OR Equals Blank for this to show in the query.
📌Note: If an employee has added an assignment but not clicked the Submit button, these won't have gone for approval either. The employee is required to click Submit to submit their assignments for approval, this option is only visible to the employee.