Service Request PO Number

In 12.1.3, we find that after entering a PO Number on the Service Request, then changing the Bill-To to another account, the PO Number is removed from the Service Request.  This causes us to request the PO a 2nd time from the customer.  How do we prevent the PO Number from getting removed?  The CSXSRISR form version is 12.0.208.12010000.21. 

Thanks, Roger