QT Event filter not working with multiple buttons

Question

I am trying to create a simple game in QT that uses event filter to catch keys pressed. It worked correctly until I implemented 'speed up' once space button is pressed. I have such a method in main window:

QSet<int> keysClicked; //declared in header

bool MainClass::eventFilter(QObject* target, QEvent* event)
{
    if(event->type()==QEvent::KeyPress)
    {
        keysClicked.insert(static_cast<QKeyEvent*>(event)->key());
        qDebug() << static_cast<QKeyEvent*>(event)->key();
    }
    else if(event->type()==QEvent::KeyRelease)
        keysClicked.remove(static_cast<QKeyEvent*>(event)->key());
    return QWidget::eventFilter(target, event);
}

and then a timer which sends keysClicked each n msecs to an object that we control.

keysClicked set should be able to contain all currently pressed keys.

And there is a problem, as it should contain spacebar, key_up, and either key_left or key_right:

1. It works correctly when I am holding spacebar, key_up and key_right
2. It doesn't work when I am holding spacebar, key_up and key_left (left key press is not found in eventFilter)
3. It works correctly when I am holding key_up and key_left

Could someone explain me what is the problem and how could I fix it? (point 2.)