Form displaying all unchecked fields

We are having a new problem with the sign-up form that started when we added a new signup list to an existing form. Now the form displays all form fields on the frontend whether they are checked in the settings or not. The problem only occurs when we select more than one list.

We have checked for plugin and theme conflicts. The PHP version is 5.5.9.

The site is hosted through WP Engine, which reports the following potential incompatibility with this plugin.

/nas/content/live/sunbrook/wp-content/plugins/constant-contact-api/vendor/guzzlehttp/guzzle/src/Cookie/SessionCookieJar.php
_SESSION on line #51: $_SESSION[$this->sessionKey] = json_encode($json);
_SESSION on line #59: if (!isset($_SESSION[$this->sessionKey])) {
_SESSION on line #62: $data = json_decode($_SESSION[$this->sessionKey], true);

/nas/content/live/sunbrook/wp-content/plugins/constant-contact-api/vendor/constantcontact/constantcontact/src/Ctct/Auth/SessionDataStore.php
session_start on line #12: session_start();
_SESSION on line #14: if (!isset($_SESSION['datastore'])) {
_SESSION on line #15: $_SESSION['datastore'] = array();
_SESSION on line #26: $_SESSION['datastore'][$username] = $params;
_SESSION on line #35: if (array_key_exists($username, $_SESSION['datastore'])) {
_SESSION on line #36: return $_SESSION['datastore'][$username];
_SESSION on line #48: if (array_key_exists($username, $_SESSION['datastore'])) {
_SESSION on line #49: $_SESSION['datastore'][$username] = $params;
_SESSION on line #58: unset($_SESSION['datastore'][$username]);

Any idea why all of the fields are displaying?