Assignments: Override grade prevents submission grade from being updated

It's not totally clear how this is possible, but we have assignment submission records for an individual assignment (not group submission) where there is a non-null grade in ASN_SUBMISSION_SUBMITTER.

The effect of this for Assignments that are not managed by the Gradebook is that it's impossible to update the submission grade, although it appears that the updated grade has been saved successfully in the Assignment UI.

The revised grade is saved in ASN_SUBMISSION, but the override grade from ASN_SUBMISSION_SUBMITTER is always used instead, even though this override grade is apparently only for group submissions (and therefore it's also not shown or editable in the UI).