Based on some server-side logging, I see that using the legacy mongo shell v5.0.2 to connect with X.509 auth sends an "authenticate" command with the $db value set to "admin". This is how I'm connecting:

> mongo --tlsAllowInvalidHostnames --tls --tlsCAFile ./testdata/certs/ca.pem --tlsCertificateKeyFile ./testdata/certs/client_combined.pem  --authenticationMechanism MONGODB-X509

Using this against a locally running mongod succeeds, but interestingly, if I try to manually run such a command once authenticated, it fails as expected:

> {"authenticate": {"$numberInt":"1"},"mechanism": "MONGODB-X509","user": "CN=x509TestClient","$db": "admin"}