In a recent update, the mechanism and the structure of the API response has been changed. Meaning, whenever you will make an API call to send a message to a user - as the API response is asynchronous, the immediate response will be HTTP_SUCCESS and you will always get the response body which includes an object with message identifier and status.
Your set callback will always receive the actual status of the message as a message-event.
Hence you will receive a bulk of event messages. Please refer to the below API documentation link for your reference https://www.gupshup.io/developer/docs/bot-platform/guide/whatsapp-api-documentation#APIResponse