Definitely ask customer support at this stage. It's probably a duplicate error between Contact and Lead and different leads are qualifying and only 1 has unsubscribed. I thought they had fixed that issue.
If the person has multiple email addresses (jhill, josh.hill, jdhill @samecompany.com) only one will be unsubscribed.
If someone fills out a form without an opt in box, they should NOT be automatically opted in (or back in).