Rest API gradeable_students endpoint underreports

[wickning1]

We have seen an issue crop up multiple times where
/api/v1/courses/:course_id/assignments/:assignment_id/gradeable_students returns an incomplete list.

Once an assignment is in this state (not sure how it gets there), it remains in this state. The fix we've found is to delete and recreate the assignment. That works.

The really fun part is that /api/v1/courses/:course_id/assignments/gradeable_students?assignment_ids=:assignmentId works perfectly.

Unfortunately when this has happened to us, the instructor urgently needs a fix, so we've been unable to leave it in a broken state for investigation. Perhaps an analysis of these two endpoints' source code could reveal the problem.