eBay has a known limitation where they only return the email on an order as long as checkout/payment is completed within a few days after placing the order. You can read more about this limitation here:
If a buyer completes the checkout process more than a few days after placing the order, their email address is returned as “Invalid Request”. If this occurs, a system like NetSuite will report an error since it requires a valid email address. If this rare scenario occurs, you’ll need to log in to FarApp’s orders dashboard and edit the order with the buyer’s correct email address so the order can import.