Hallo zusammen,
die Antwort auf Eure Fragen steht im Hinweis 2058854 (leider nur auf Englisch):
For a stock transport order, it is standard behavior that the partners are not copied from the vendor master.
This is because normally partners are tied to a vendor. In a standard PO the partners are defaulted into the document. However in a stock transport order, there is no vendor present. So partner determination is not automatic like that in a standard PO. Please kindly review SAP note 117537 which explains the logic.
The partners are always assigned to a vendor; during a standard purchase order, the partner determination proposes the partners from the master record of the vendor from which you order. However, during a (company code internal) stock transfer no vendor is involved, therefore there is no possibility to carry out a partner determination.
Resolution
You have to maintain partner manually in the purchase order.
As a workround you can use the Business Add-In (BAdI) ME_PROCESS_PO_CUST, which enables you to extend the business logic of the Enjoy purchase order on an individual basis.