There is currently an issue with some multi-line orders. When retrieving the open order, it can happen that we don’t immediately give back all orderItems in the response. With a (small) delay we do give back all orderItems from the orderId. We are working hard to resolve this issue. For now, we advise you to retrieve the orderId again just before confirming/shipping an order, to check whether any orderItems have been added in the meantime.
We apologize for the inconvenience. We will update this page when the problem is resolved.