Explanation
room messages are considered sensitive information and since Webex initiated the request to your backend, it did not have your Access Token with which to decrypt the message. In order to get the sensitive information, your app needs to use the resource id to fetch the full resource. Using the above messages example, your app could fetch the complete message object along with the text by doing an authenticated (via your Bearer Token) GET request to /messages/{id}
https://developer.webex.com/docs/api/guides/webhooks