Before, in all other channels except Messenger and Instagram, if you send a message from the inbox, the message is immediately shown in the inbox. This behavior was causing many problems because, many times, the message failed to be sent for many reasons, and the admin would think the message was sent. You will only know that a message fails to be sent if you reload the inbox because the message you sent will disappear.
Now messages are shown in the inbox only if successfully sent.