Encumbrances in Payables

This is regarding one of the encumbrances issue we are facing in R12.1.3

1) When we create PO for expenses, system is generating encumbrances correctly.

2) But when we create prepayment invoice and match to the PO, system is reversing the expense encumbrances and creating actuals with advance account.

Due to this after i create advance invoice and match to PO and pay the invoice, system is showing zero encumbrances for that expenses allowing users to create more PO's and invoices with same expense account even there are no funds in absolute controll.