Discard Line Does Not Reverse Invoice Encumbrance

We use encumbrance accounting.

Requisitions = Commitments

PO = Obligations

Invoice = Invoices

Our users have a business process need to reverse and/or discard invoice lines matched to a PO.

We've tried discarding the invoice line and while the invoice is no longer matched to the original PO; the encumbrance journal entry reversal is not generated by Payables.  And the purchasing application's 'Amount Billed' field is not updated.

Any ideas?

Thanks,