Since the introduction of Order number field where you actually select sales order, the free text Order number has been obsolete.
In the latest version, the field is visible only on invoices where there was something previously entered. But I really want to discourage use of this free text field for new invoices. I mean there are two Order number fields which is rather confusing.
Any reason you can’t use proper linking between your sales invoice and sales order using the new Order number field which is next to the Customer field?