Multiple Modules - Routing: Default routing not working when specifying route opposed to as in single

I have setup my web app to use multiple modules.

I have root index with {{ content() }} to view through to the controllers view.

By default with no route defined when I go to https://myhost.com/invoiceController then the views index and controller view will yield normally.

However when I define a route:

    $router->add(
            "/invoiceController",
            array(
                'module'     => 'frontend',
                "controller" => "invoice",
                "action" => "index"
            ));

Then the root index view is completely bypassed with only the local controllers index view being output.

What gives?