Previously unsubscribed guest/customer is not resubscribed when selecting groups on success page

[bexdoggett]

Issue Description

If a customer or guest has signed up for the newsletter and then unsubscribed, when they place a new order and land on the success page they can still see the groups block from Mailchimp. If they select a group and click update their groups are updated, but they are still unsubscribed. If a customer or guest who has never signed up to the newsletter follows the same steps they are subscribed to the newsletter.

Preconditions

1. Magento CE 2.1.14 (With a fix to stop the order.success.additional.info from outputting twice)
2. mc-magento2 1.0.28
3. Newsletter group created with interests, set to display as checkboxes
4. Any browser

Steps to reproduce

1. Sign up for the newsletter
2. Unsubscribe from the newsletter
3. Add any product to the cart and checkout using account or email address from above
4. Once on the success page select a group and submit

Actual and Expected result

Expected result:
When looking in the newsletter subscribers table, I expect to see the user resubscribed to the newsletter. Alternatively the groups block should not be available on the success page for unsubscribed customers/guests.

Actual result:
User is still unsubscribed.