Yii2 HTTP session on build server - unit-testing

Question

I'm attempting to get our PHPunit tests to run on our build server (Windows jenkins server).
They run on my local Ubuntu 16.04 system correctly, but when I get over to the windows build server I'm having some weird session issues.
I have a light wrapper around the Yii2 Session class that we use so that if we change frameworks I only need to update the session calls in one place.
Here's that class:
<?php
use \yii\web\Session AS Session;
class SessionHelper {
public static function get($key, $defaultValue = null) {
$yiiSession = new Session();
return $yiiSession->get($key, $defaultValue);
}
public static function set($key, $data) {
$yiiSession = new Session();
return $yiiSession->set($key, $data);
}
} //EOF
I'm using this SessionHelper for all things session in my app and inside my Test classes.
The issue i'm running into is this: when I call SessionHelper::set('something', 'something'); from my test case it will correctly set the property into the session. However, the very next time I call SessionHelper::set('something-different', 'something-different'); it loses all previous session data and only has this new session field.
Is this due to the generation of a "new Session();" in each get/set request? I checked and the session settings are identical between servers and the session_id() is the same all of the time.