Qt HTTP GET request works, but POST doensn't

Question

I want to send an HTTP Post request once a user type an incorrect password. I am sure that my code other than the HTTP request I send works, however when I try to send a GET request it works, but POST request doesn't. I suppose it is connected to the Qt lifecycle. I have tried various of things, but nothing worked.

Here was my initial code.

#ifdef __cplusplus
extern "C"
{
#endif

    JNIEXPORT void JNICALL Java_tech_secureme_SecureMeDeviceAdminReceiver_handleHttpRequest(JNIEnv env, jobject obj, jint n)
    {

        QNetworkRequest request(QUrl("https://secureme.live/incorrect_password"));
        request.setHeader(QNetworkRequest::ContentTypeHeader, "application/json");

        QNetworkAccessManager *manager = new QNetworkAccessManager;

        QObject::connect(manager, &QNetworkAccessManager::finished,
                         [](QNetworkReply *reply)
                         {
                             if (reply->error())
                             {
                                 NotificationClient::getInstance()->setNotification("err", reply->errorString());
                             }

                             reply->deleteLater();
                         });

        QJsonObject json;
        json["key"] = "value";
        QJsonDocument jsonDoc(json);
        QByteArray jsonData = jsonDoc.toJson();

        manager->post(request, jsonData);
    }

#ifdef __cplusplus
}
#endif

This JNI function gets called whenever user types his password incorrectly. Again I am sure that this part works.

Why does GET request works and POST doesn't ? How can I fix that ?