The past due amount is zero for sponsorships even when there is an outstanding past due payment.
We are currently evaluating this issue for a fix in a future release.
Steps to Duplicate
1. Go to a sponsorship record that has a recurring payment that is past due. 2. Go to the Payment tab of the sponsorship 3. Note that the Past due field shows $0.00 even though the Next transaction date is a date in the past.