If you code an expense item on an expense receipt to a purchase order and then link that receipt to a credit card transaction, the PO coding is removed from the expense item.

This is expected behavior. Initially, the purchase order that you coded to the expense item was written to the vendor associated with your employee number.

When you matched the receipt to a credit card transaction, the vendor changed to the credit card vendor. As a result, the initial purchase order information was cleared from the expense item, allowing you to code that item to a purchase order that was written to the credit card vendor.