Lineitems and OrderForms not always returned in same order

Description

The order of OrderForms, Shipments, LineItems in an OrderGroup is not guaranteed to be the same every time. In rare scenarios, they might change and cause problems.
For instance, the order system relies on the index of LineItems in a OrderForm to distribute them into shipments. So, a change in the order of line items changes shipments so that they contain wrong items and/or quantities.