added a comment - - edited
Note that this bug only occurred when you were updating the grades of a single user and they had a raw grade of null.
1) make sure
MDL-25718 has been integrated.
2) create a new lesson activity. dont access it as a student to ensure no students have a grade for it.
3) unenroll a student from the course. course administration -> users -> enrolled users. make sure you remove the users enrollment and all grades. dont just suspend their enrollment.
4) reenrol them on that same page making sure to recover their grades.
5) go to the grader report in the gradebook and check that the student still has a null grade (a dash) for that lesson.