Exacly, prior to 8.6, the HTTP Webhook Connector supported a response body expression. As of 8.6, this was replaced with a more powerful construct that allows control over not only the response body, but also the headers and the HTTP status returned by the Connector.
I think this has some to do with the configuration of the kubernetes environment and istio. But we did not found the missing config. Are you able to help us with that?