Profile is 'Special' and not a normal Basic Form, basically you need to adjust the Form action by adding something like the following to the 'Profile' Web Page html
<script>
$( document ).ready(function() {
// after submit will send the user to the returnUrl
$("#content_form").attr('action',"/profile/?returnUrl=%2Fforums%2F");
});
</script>
Similarly if you always want the users to land on a specific page after sign in you can adjust the Header web template and hard code in the returnUrl parameter value on the Sign in link.
