If you have created an expense item, coded it to a purchase order, and then linked it to a credit card transaction, the purchase order will disappear from your expense item.

This happens because the purchase order entered before linking the expense to a credit card transaction was written to the vendor associated with your employee number.

When you matched the credit card transaction, it changed the vendor to the credit card vendor, and the purchase order was cleared so that you could code the expense to a purchase order that was written to the credit card vendor.