This is very likely due to the Enroll Date missing from the Student Enrollment row. To put the Enroll Date back:
  1. Navigate to Core > People Finder - look up a student
  2. On the Access tab, click the Persona (i.e., Alum) to get their school enrollment record
  3. In the header bar, click the edit pencil
  4. Select Edit school enrollment
  5. In the Enroll date field for their first enrollment row, enter the enroll date
  6. Click Save