Yes you are observing standard SAP behaviour. It is unclear from your question as to whether you are using standard SAP code or have employed your own code and/or user exits.
I don’t recommend you use POSEX to identify the line item in the order. Better to use the Customer Part Number (KDMAT). We have similar problems when matching Sales Orders (although in our case we use Schedules – standard practice in the Automotive Manufacturing Supplier Industry).
We successfully match many Customers -> Many Schedules -> Many Order Lines but have gone away from standard SAP, mainly due to the limitations you are experiencing.
Good Luck – Ed.