Hi Nimrod,
for PI/PO to ECC communication (inbound to ERP) the receiver fields are optional. So the IDoc Adapters usually insert your PO system values in these fields if you do not override it. Inbound IDocs were just processed by the configured program in the partner profile entry for SNDPRN and type.
For outbound communication the ECC will execute the standard ALE behaviour, so you will always have your ECC system in the sender related EDI_DC40 fields.
In any case let us know what SAP support answers you.