This seems like a caching issue, so i have to ask – are you using any caching plugin? If you do, try to deactivate it and let’s see if after that everything will work fine.
So, each time a user subscribes, he receives a unique ID. You can see his ID if you go to PMS –> Members –> Edit Member Subscription –> in the page URL, at the end of it, you’ll se the ID == a number. That’s how to check it.
Regarding the MailChimp situation, we do have a very good integration with it.