Subscribers removed from previous lists

I am using your plugin on a WordPress multiuser site. Subsites are location-based and each subsite has a signup for their local newsletter, which is configured in the Signup Widget Settings to go to their locations contact list in Constant Contact (we have one Constant Contact account and each location has their own list within that account). For instance, on the website for City A there is a web sign up form where people can subscribe to City A’s newsletter, and the same goes for City B and City C. The problem is that when someone subscribes to City A’s newsletter, Constant Contact shows that they are no longer subscribed to lists that they were previously subscribed to. For example, if someone is subscribed to City A’s newsletter and they sign up for City B’s newsletter, Constant Contact will show that they are only subscribed to City B’s newsletter. There are many cases where cases where people are signed up to receive newsletters for multiple cities and we are losing this information as the signup form seems to be overwriting their previous subscription preferences.

Is there a way to resolve the issue so that subscribers don’t lose their previous subscriptions while using the web sign up form? I am using Version 2.4.1 of your plugin with WordPress 3.8.3.

I saw that others were having this issue and that it was fixed in a previous version, however the problem still persists for me. http://ww.wp.xz.cn/support/topic/plugin-contact-form-7-constant-contact-subscribers-are-removed-from-list-1-after-using-list-2-form?replies=12

https://ww.wp.xz.cn/plugins/constant-contact-api/