Supplier Payment Method

The field IBY_EXTERNAL_PAYEES_ALL.DEFAULT_PAYMENT_METHOD_CODE is updated only when a payee is created. This is automatically updated and there is no UI for this field.
If a user updates the default payment method at supplier or supplier site level, then the payment method is updated in the IBY_EXT_PARTY_PMT_MTHDS.PAYMENT_METHOD_CODE.

Column IBY_EXTERNAL_PAYEES_ALL.DEFAULT_PAYMENT_METHOD_CODE is never used for any kind of defaulting. It just serves as a place holder for AP supplied
parameters and populated only at the time of creating payee, remains dormant thereafter.

Actual default payment method is evaluated from active records of table IBY_EXT_PARTY_PMT_MTHDS where it is filtered by the primary_flag (should be 'Y') and by the external payee id of the Site,AddressOU,Address,Supplier
level payee (in the order of preference).