Drop move that will never be linked to drop shipment
In a particular case, the sale create a drop move that will never be linked to a drop shipment:
If the linked purchase line has an shipment exception that is ignored, the sale goes into exception. If the user chooses to recreate the move, it will recreate it as a drop move. But the purchase request is done and also the purchase line, so it will never be linked to a drop shipment.