NativeSQLQueryConstructorReturn is not processed by SQLQueryReturnProcessor

I am trying to call a stored procedure with 2 OUT parameters. I have declared a NamedStoredProcedureQuery referencing a SqlResultSetMapping:
{{
@NamedStoredProcedureQuery(name = "set_handler", procedureName = "API_CUSTOMER_PKG.set_handler",
parameters = {
@StoredProcedureParameter(mode = ParameterMode.IN, name = "p_agreement_id", type = String.class),
@StoredProcedureParameter(mode = ParameterMode.OUT, name = "p_success_o", type = Integer.class),
@StoredProcedureParameter(mode = ParameterMode.OUT, name = "p_error_message_o", type = String.class)
},
resultSetMappings = "procedure_result_mapping")
@SqlResultSetMapping(name = "procedure_result_mapping", classes = {
@ConstructorResult(targetClass = ProcedureResult.class, columns = {
@ColumnResult(name = "p_success_o", type = Integer.class),
@ColumnResult(name = "p_error_message_o", type = String.class)
})
}
}}

The stored procedure is executed and produces a Map of the two output parameters. Then SQLQueryReturnProcessor.processReturn is called, which calls processConstructorReturn. But this method is empty, so the SqlResultSetMapping has no effect, and the Map is returned as the query result.

Is this by design, or is the functionality just missing?