The "Don't opt-out of interest groups: only opt-in" option is present in the Signup form, but is not in the Mailchimp Subscription field that can be added, for example, to a profile registration page.
This way, if the contact is already present in the Mailchimp audience and then create an account in Drupal without selecting to be subscribed (legitimately thinking that he is already) or select any group, his contact is automatically achieved in Mailchimp and he is not receiving emails anymore.
Having the option "Don't opt-out of interest groups: only opt-in" available in the Mailchimp Subscription field as well will avoid this issue.
Closed: won't fix
2.0
Lists Module
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
Thanks for submitting this ticket — and sorry we didn’t get back to you about it sooner. We are cleaning up old issues and issue for old versions and are going to close this one. If this issue is still relevant for you, please re-open it and update the “Version” field, and include any updates to the behavior you are seeing.With a smaller issue queue, we’ll have an easier time getting back to you.