In Sales Order, selecting a Customer with Term preference will auto populate the terms field under Billing Tab. However, when following up with selecting a customer with no term preference, the term field will not be cleared, and will stay fixed as the previous term set when selecting the first customer with the term preference.
It is a standard behavior that the term field for Sales Order is not cleared when selecting a customer with no terms preference. This field must be manually removed.
Alternatively, a workflow can be created to clear the field if the customer does not have a term preference.