I had the issue previously where members are not automatically added to MailChimp list. Contacted support and they provided a fix that was working fine. Until I recently to 1.1.5.
The issue is, if I set the ad-on to add "Registered users mailing list (not members)" it works fine. And registered users are added to the list. But if I set "Members mailing list" option, nothing is added to mail list after user register.