When creating an estimate, it asks you to select a customer in the top field. When this is copied to a purchase order, as this is an order you would submit to a supplier, the field now asks for a supplier name. This therefore means the field will not auto-fill with a customer name, as it is instead asking for a supplier to be entered. When copying an estimate to an invoice, this box will be auto-filled with the customer's name.
Please let us know if you have any further questions.