OWA issues redirects with Status 304 (Not Modified)? That seems odd.
I’m assuming if that’s the only transaction, the connection was initially made over HTTPS. If the request was successfully proxied to OWA, that means the certificate was definitely received by the client, because that needs to be negotiated before the request is sent to begin with. How else would the tool be checking for a certificate? Does it expect OWA to provide one rather than a TLS termination proxy?