Android foreground service with Qt stops working when the main application has been closed

I'm working on android 7.0 and I'm trying to write an android service using Qt.

Basically what I'm trying to do is a simple service who replies to commands received on ZMQ and for this task I'm using a foreground service since I want that the service continues to run even if the application that has started it has been closed.

This is my java service:

    @Override
    public void onDestroy() {
        super.onDestroy();
        Log.i(TAG, "Destroying Service");
    }

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        super.onStartCommand(intent, flags, startId);
        new Thread(new Runnable() {
                        public void run() {
                            startZMQ();
                        }
                    }).start();


        return Service.START_STICKY;
    }

    @Override
    public IBinder onBind(Intent intent) {
        return null;
    }
}

I started the service in this way

JNINativeMethod methods[] {{"startZMQ", "()V", reinterpret_cast<void *>(startZMQ)}};
QAndroidJniObject javaClass("org/qtproject/example/qtandroidservice/QtAndroidService");

QAndroidJniEnvironment env;
jclass objectClass = env->GetObjectClass(javaClass.object<jobject>());
env->RegisterNatives(objectClass,
                     methods,
                     sizeof(methods) / sizeof(methods[0]));
env->DeleteLocalRef(objectClass);

QAndroidIntent serviceIntent(QtAndroid::androidActivity().object(),
                                    "org/qtproject/example/qtandroidservice/QtAndroidService");
//serviceIntent.putExtra("name", name.toUtf8());
QAndroidJniObject result = QtAndroid::androidActivity().callObjectMethod(
            "startService",
            "(Landroid/content/Intent;)Landroid/content/ComponentName;",
            serviceIntent.handle().object());

and this is the zeromq server function:

static void startZMQ(JNIEnv *env, jobject /*thiz*/)
{
        void *context = zmq_ctx_new ();
        void *responder = zmq_socket (context, ZMQ_REP);
        int rc = zmq_bind (responder, "tcp://*:5555");
        assert (rc == 0);

        while (1) {
            char buffer [10];
            zmq_recv (responder, buffer, 10, ZMQ_NOBLOCK);
            printf ("Received Hello\n");
            sleep (1);          //  Do some 'work'
            zmq_send (responder, "World", 5, 0);
        }
}

The service works fine until I keep the application open but when I close the application (but the service keeps running since it is a foreground service) the service stops to work (from my client application I don't see any reply to my requests using zeromq).

I tried exteding from QtService instead of Service but the app randomly crashes and the service never works (I also do not see the foreground service icon so I think that there is a problem starting a QtService as foreground service).

What I'm doing wrong? How can I obtain a working service (with the main application, which launched the service, closed) who correctly replies throught zeromq?