a colleague of mine informed me that I must press the "Create" button before leave the order (I press only the "Save" button). In this way I can add other products re-entering in the order.
Now it works, but I think that an application warning should appear (or the order should be removed at all and not leaved in a "broken" status).
Now a more technical question; with this query:
select * from wp_atum_inventory_orders poi left join wp_woocommerce_order_items oi on poi.order_item_id=oi.order_item_id;
I could retrieve the ordered products, but I note that the one who belong to POs have the right part of the join to null. I need to read the "order_id" also for PO. How can I do this?