Invalid login returns 200 HTTP code

I’m calling an API in my ServiceStack WS that requires authentication. When I enter the correct credentials, it works fine. When (on-purpose), I enter an invalid password, nothing happens (using curl). Running this in Postman, I see status code 200 and the response body is a long HTML code which seems to be the form to enter the credentials.
Obvisously, I’m not running this interactively so that doesn’t help. I was expecting 401 error. What am I missing?

Please provide the full HTTP Request/Response Headers for any integrations issues like this.