I would like to know:
Was the card completed: On, Before, or After the due date? I'm not sure where the card should exist in the calendar: (on the due date? or on the completed date?).
What I do know is if you're tracking data; Due date and completed date are not the same things. On a good day, they will coincide but it will often workout that the completion will happen on a day that is different than the "due date".
I feel this is a flaw that should be fixed. In the meantime, I may look into using Butler to work on this.