Unable to update SupplierTaxRegistrationNumber on AP Invoice

Summary:

Unable to update SupplierTaxRegistrationNumber on AP Invoice after creation (AP-810908), despite GUI allowing it

Content (please ensure you mask any confidential information):

The REST API for AP Invoices returns error AP-810908 (AP:::AP_RS_UPD_ATTR_INV_SAVED — "You can't update the Supplier Tax Registration Number attribute once the invoice is created") when attempting to PATCH the SupplierTaxRegistrationNumber field on an existing invoice.

The same field is fully editable via the Fusion UI for the same invoice, in the same state, as the same user. Users can select a new value from the dropdown LOV on the Invoice Header → Accounting tab and save it successfully.