It appears that the python client's version of gRPC isn't using standard TLS (no SNI, no ALPN), and therefore, it can't be routed through an application gateway.
How do I turn it off?
Omitting the connect_to_custom grpc options and/or providing empty values results in errors.
Alternatively, is there a way to enable standard TLS that includes SNI and ALPN (which the http client does)? Or provide a pre-encrypted connection to use rather than relying on the internal mechanisms?