TLS proxies are easy to fingerprint. The remote end (or intermediate routers) may not be able to deduce that it is in fact Mitmproxy that is running, but at least it can see a non standard ja3 fingerprint. Depending on that it might take different actions on what to show/do to the client.
Some way to customize the client hello (look like the one from chrome, firefox ...) would be awesome :)
afaik it would be the first proxy solution which does that.