Updating a supplier's payment method does not change the 'payment_method_lookup_code' in 'ap_supplie

If we update a supplier's payment method and query the field 'payment_method_lookup_code' in 'ap_supplier' table, it only contains null value. Furthermore, the said changes does not update the 'last_updated_by' and 'last_update_date' field of the above mentioned table. Same is true for production and test instance. Could this be a bug or is the behavior normal? If so, where can we get the value for the payment_method_lookup_code field?

 

Step to change supplier payment method:

Payables > Suppliers : Entry > (select a supplier) > update > (go to payment detail) > (select one of the payment methods)