Hi Michel,
We have recently used WIP batch functionality for external processing at vendor site and we never saw this error. This step is one of the operation/phase in process order.
We have used MITO_TR to send the material out to vendor and MIGO_GR to receive against purchase order. Once we received the WIP Batch it goes against process order and we will not see in MMBE.
Can you explain your scenario in details? Then I may be able to help you.
Thanks,
Ramesh