Why is PO for alternate vendor sourcing not retrieving proper Supplier Item?

 In R12, when I run Autocreate for a requisition and source to a non-primary vendor, the Supplier Item on the PO references the primary and not the alternate vendor.  Supplier Items are stored in the ASL attributes, so need to figure out why the correct item is not being defaulted on the PO line.  There is no easy workaround, since Supplier Item is a free-form field and you can't manually update it from a LOV.

thanks,

Ken