Are there any other plugins on your site that are setting a login redirect? Before the WP Discourse plugin redirects the user to the URL provided in the redirect_to parameter, it calls the wp_login action. If another plugin is hooking into this action to redirect users, it will always take precedence.
Unless another plugin is overriding the behaviour, it will always redirect the user to the URL you have added to the redirect_to parameter. If there is a redirect in place from that URL to another one, it will be applied.