Prevent BPA Supplier Item default

Blanket Purchase Agreement "PA-DEF".

Supplier "ABC".

Supplier Trading Partner Item Relationship for our item "123 "; Supplier "ABC"; Trading Partner Item "456".

When I define item "123" on BPA "PA-DEF"; the Supplier Item Column defaults "456".

We do not want "456" to default and want it to remain null. We maintain the trading partner items on the Item Master and/or ASL. When a change is made; the old trading partner item on the item master is end dated and status updated to obsolete. The new trading partner item is not updated on the BPA; therefore, we'd prefer to allow the PO to populate the Supplier item using the hierarchy directly from the ASL and/or Item Master.