Qt::UniqueConnection not working with lambda slots

Consider the following code:

public slots:
   void testSlotA();
signals:
    void testSignal(int x);
private:
    int m_x = 0;

void MainWindow::testSlotA()
{
m_x++;
qDebug() << "?? PING" << m_x;
connect(this, &MainWindow::testSignal, this, [=](int x)

, Qt::UniqueConnection);

emit testSignal(m_x);
}

Now if I call testSlotA 3 times it produces the following output:

?? PING 1
!! PONG 1
?? PING 2
!! PONG 2
!! PONG 2
?? PING 3
!! PONG 3
!! PONG 3
!! PONG 3

This is wrong, using Qt::UniqueConnection the output should be:

?? PING 1
!! PONG 1
?? PING 2
!! PONG 2
?? PING 3
!! PONG 3