The Customer information (salesperson, tax group, price list, etc.) populating the Order is different than the Customer information in Accounts Receivables / Customers.
Check if the Ship To field on the Order is populated and then check the AR Customers / Ship To Locations screen for the customer. When a customer is selected for an order, all the default values in the customer record are used to populate the order entry fields. However, is a Ship To Location is specified, all the information is over-written with the default values from the Ship To Location.
If you need to update settings for the customer, ensure that all of the relevant Ship To locations are also updated.