UNASSIGNED PURCHASE REQUISITIONS NOT PUBLISHED IN R3

Hi gurus

we're implementing the following scenario:

1. Subcontractor MRP area
2. Direct procurement of the component from the vendor to the subcontractor

the flow is properly planned and the purchase requisitions, with the right source of supply, are published in the MRP area of the subcontractor when transferring to APO the vendors of the components and manually creating the transportation lanes between vendors and subcontractors;

ISSUE: we neither want to transfer the vendors of the components nor we want to manually maintain the transportation lanes, leaving the purchase requisitions, in APO, unassigned to any source of supply because the vendor is going to be manually entered in R3; such a purchase requisition is created by the SNP planning run but we cannot publish it and the delta report neither detects any inconsistency.

Is there someone who can tell me why?  

Thanks in advance, greetings, Marco.