Merge Purchase Order + Keep Production Order Link

PROBLEM:  When merging Purchase Orders that are automatically generated from Production Orders, the Purchase Order is "de-linked" and removed from Related Orders under the Production Order.  The Production Order cannot be completed.

EXAMPLE:  We require Co-Manufacturers to complete 'finishing' on our products.  We will send 30-50 parts at a time to get painted.  A Production Order needs to be created for each part, which in turn produces an individual Purchase Order.  Rather than send our Paint Service provider 30-50 Purchase Orders, we merge them into one.  However, once they're merged, they lose the link to the Production Order and it cannot be completed.