Why can't Quantity and UnitSellingPrice be updated via AR Invoice REST API? (AR-857833)

We are working on updating invoice lines in RA_INTERFACE_LINES_ALL table in Oracle Fusion using the Receivables Invoice REST API. While trying to update the Quantity and UnitSellingPrice for existing invoice lines, we are receiving the following

Error Messages:

The Quantity attribute can't be updated using this REST service. (AR-857833)
The UnitSellingPrice attribute can't be updated using this REST service. (AR-857833)

Context:

• We are using the standard /fscmRestApi/resources/11.13.18.05/autoInvoiceInterfaceLines/{InterfaceLineGUID} REST endpoint.
• We attempted a PATCH call to update these fields.
• There seems to be no alternative via SOAP or PL/SQL, and this behavior is blocking a key requirement.