by Ken Task.
In Moodle 2.7 the old assignment is considered 'legacy' (for lack of a better term) and all assignments should have been converted to the the new assignment mod which uses table names using the following convention: mdl_assign_whatever. Yes, mdl_assignment_submissions should contain no data ... but mdl_assign_submissions should.
Was investigating something else today related and discovered that.
Do you have any assignments in the system of the older version?
'spirit of sharing', Ken