OpenIdConnect signin-oidc route not handled by ASP.NET MVC

For anyone browsing this in the future, this is the answer:

Owin.OpenIdConnect does not support "code" only ResponseTypes. You need to set "id_token" too. If, for any reason, you cannot do this, you will basically need to implement parts of the spec yourself (mainly by hooking up into the MessageReceived Notifications Event).

See this part in the source code of the OpenIdConnect Handler:

https://github.com/aspnet/AspNetKatana/blob/0f6dc4bf2722fb08759da3eacaf38f2a098771bd/src/Microsoft.Owin.Security.OpenIdConnect/OpenidConnectAuthenticationHandler.cs#L258-L264