Unenrolled students' assignment submission files never deleted

When a student uploads a file to an assignment and then the student is unenrolled from that course the submission file is not deleted from the moodledata, although it isn't available any more.

I understand that the clean up should not be done just when the student gets unenrolled (to avoid data loss in wrong unenrollments), but I think these files should be deleted some (reasonable) time after unenrolment.

This is something that happens to us because we usually reuse courses, but I don't know if this is something deliberate and we should reset all the courses' assignents before reusing courses or (as I think) it's a bug as far as these files are never going to be used again.