AJAX and Session Persistence not working

Hi Laracastians, I'm working on building a little compare bin and I'm having an issue with persisting values to session storage with AJAX requests. Here is my method in my controller

public function postAddPersistence() {
        $postData = \Request::all();

        $pattern_compare_persistence = \Session::pull('pattern_compare_persistence', []);

        $already_in_array = array_search( $postData['pattern_slug'], $pattern_compare_persistence, true );

        if($already_in_array === false) {
            $pattern_compare_persistence[] = $postData['pattern_slug'];
        }

        \Session::put('pattern_compare_persistence', $pattern_compare_persistence);

        \Session::save();

        return \Response::json(\Session::get('pattern_compare_persistence'));
}

And here is my AJAX call (I'm using vue.js)

if(add_to_persistence) {
    var postData = {pattern_slug: pattern_slug};
    this.$http.post('/api/patterns/add-persistence', postData, function() {}, function(error) {});
}

Now, the issue is that things seem to be working intermittently. If I print_r(\Session::all()); right before the return statement, I will see the 'pattern_compare_persistence' key with the newly added pattern_slug correctly in the array. But when I check the persistence, the newly added pattern_slug is not actually in the array.

What is breaking my brain a little bit is the fact that when I do a test via the Postman chrome extension, the values will be added to the session persistence perfectly. Am I doing my AJAX call incorrectly? Am I not returning correctly in order for the Session to save properly?