Please see this forum post for more details: https://moodle.org/mod/forum/discuss.php?d=392990

In short:

1. Make sure 'grade to pass' is set on the assignment (something like 50 for example). Also set 'attempts reopened' to 'automatically until pass'.
2. Attempt one -> grade is pushed to grade book
3. New attempt is opened -> grade -1 is written to assign_grades for attempt 2. Grade book keeps grade for attempt one (grade_grades)
4. From now on, the grade book only looks at the grade for the last attempt for this assignment. Which is attempt 2. But that currently has grade -1 so nothing happens to the grade book. The grade book is never monitoring what is entered in attempt 1 for the assignment, as it is irrelevant for the grade book: that should only store the grade with the highest attempt number

This gives the following bug:

1. Teacher grades attempt one and opens attempt 2 (or it is opened automatically when assignment attempts is set to 'automatically until pass')
2. Teacher made a mistake in the grade for attempt one and corrects the grade. Or, student and teacher come to conclusion that the grade for attempt one needs to be adjusted
3. Grade book wil never pick up this grade change for attempt one, resulting in a grade book that gives the wrong information.