Integration with MailchimpWP works only when page reloads

I’ve tried to add this code `<label>
<input type=”checkbox” name=”mc4wp-subscribe” value=”1″ /> Subscribe to our newsletter.</label>` with action ‘um_after_from_fields’ to the registration form to use it with the Mailchimp for WordPress plugin since they have the option to integrate with custom forms.

The problem is that if the user registers and checks that checkbox he IS NOT subscribed to Mailchimp.
BUT if the user tries to register and makes a mistake (causing a page reload) and then successfully registers (with that Checkbox checked), the user IS subscribed to Mailchimp.
What is going on here and how can I make it work for everyone – even those users that don’t make mistakes?

Btw. On the default WP registration page, it works every time.

I would really like to make this work, please help.