Hi,
Could you clarify if the API request generated the ManagedCustomerServiceError.SAVE_CUSTOMERS_FAILURE error, but then, when you checked it via the UI, the creation was successful? If so, then this seems odd since if the API request failed, then, there should be no data being reflected in the UI for this request. With this, then kindly provide your clientCustomerId and the complete SOAP request and response logs so that I can further check on it. Kindly reply via the Reply privately to author.
Best regards,
Luis
AdWords API Team