App not redirecting to proper page upon receiving server response

First, I am sorry for the odd name of this thread but I was not sure how to name it. I am working on a NodeJS app and I seem to have stumble upon a bug I really don't understand. This is an eLearning app so, at some point, an admin will create quizzes and questions for other users to answer. The one creating a quiz also has the possibility to duplicate a quiz and edit the title and description of the quiz. For this I created a route, /duplicatetest.js, that has a GET and a POST method. The GET method brings the quiz data from the server and sends it to front end for display:

app.get('/duplicatetest/:idTest', isLoggedIn, hasPermision, function(req, res) {
    console.log(currentDateTime.getCurrentDate() + " - GET > duplicatetest");
    customLogging.customLogging("GET > duplicatetest");

    var idTest = req.params.idTest;

    getTestToDuplicate.getTestToDuplicate(idTest)
        .then(function(result) {
            res.render('duplicatetest', {
                'test': result
            });
        });
});

The POST method saves the changes to DB:

app.post('/duplicatetest', isLoggedIn, hasPermision, function(req, res) {
    console.log(currentDateTime.getCurrentDate() + " - POST > duplicatetest");
    customLogging.customLogging("POST > duplicatetest");

    var newTest = req.body.newTest;

    saveNewTest.saveNewTest(newTest)
        .then(function(result) {
            console.log("Sending response now...");
            res.send({
                "status": 201,
                "message": "",
                "value": ""
            });
        });
});

Both seem to be working just fine. On front-end I have the following JavaScript:

$(document).ready(function() 
{
    $("#testName").val(test.title);
    $("#testDetails").val(test.testDescription);

    var url = window.location.pathname;
    var idTest = url.split("/");

    $("#nextStepTest").click(function()
    {
        const token = getCookie("u.tkn");
        test.token = token;

        var title = $("#testName").val();
        var details = $("#testDetails").val();

        var modifiedTest = {idTest: idTest[2], title: title, description: details, author: token};
        var newTest = {newTest: modifiedTest}

        $.ajax({
            traditional: true,
            url: '/duplicatetest',
            type: "POST",
            contentType: 'application/json',
            data: JSON.stringify(newTest),
            dataType: 'json',
            success: function (response) {
                if (response.status == 201) {
                        window.location = "/tests";
                    }
                },
            failure: function (response) {
                console.log(response.message[1]);
            }
        });    
    });

    function getCookie(name) 
    {
        const value = `; ${document.cookie}`;
        const parts = value.split(`; ${name}=`);
        if (parts.length === 2) return parts.pop().split(';').shift();
    }
});

However, I encounter the following behavior:

I visit /duplicatetest/idTest and make the changes. Say I have a test with title Test 1 and description Test Description 1. I change them to Test 1 Duplicate and Test description 1 Duplicate. On saving, the data is saved in the DB without problems but instead of receiving a response and being redirected to /tests, the page refreshes, displays the original data (Test 1 and Test Description 1) and the url changes from /duplicatetest/idTest to /duplicatetest/idTest?testName=Test+1+Duplicate (or whatever new name you pick for the new quiz).

On rare occasions however, the redirect works. I am using similar ajax calls for other pages and situations, without any problem.

What I noticed so far is that mostly there is no response from the server in front-end but so far I could not explain why.

What exactly am I doing wrong here and where is that "?testName=Test+1+Duplicate" coming from? I would really appreciate any suggestions since I've been trying to find an explanation for this for some time now...