The mock log request body does not show data when the request content-type header is application/x-www-form-urlencoded; charset=utf-8. The problem appears to be charset=utf-8 being included in the content-type. When removed the request body appears.
I believe this is a problem with the Postman mock log view trying to auto-format the data. Is there any way to see the unformatted RAW request body in the log instead? For comparison, when the request is sent to a hookb.in endpoint with charset=utf-8 in the content-type, I can see the RAW request body key/value pairs just fine. The request is being sent by a Twilio webhook, so unfortunately I have no control over their content-type header.