Membership 2 MailChimp List Issue

When a user signs up for any specific membership plan, he is not added to the proper MailChimp list. For replicating the issue, please enable the MailChimp Integration Addon.
Then edit any membership plan and assign a MailChimp list in the "Select a list" inside the Details tab.

Users are not being added to the specific list when they sign up for a membership.