What concurrency issue arises from the way VisibilityDerived handles the ‘flag’ variable?

class VisibilityBase {
    protected volatile boolean flag = false;
    public void toggle() {
        flag = !flag;
    }
}

class VisibilityDerived extends VisibilityBase implements Runnable {
    // Shadowing the volatile flag
    public boolean flag = true;
    
    @Override
    public void toggle() {
        flag = !flag;
    }
    
    public void run() {
        for (int i = 0; i < 1000; i++) {
            toggle();
        }
    }
    
    public static void main(String[] args) throws InterruptedException {
        VisibilityDerived vd = new VisibilityDerived();
        Thread t1 = new Thread(vd);
        Thread t2 = new Thread(vd);
        t1.start();
        t2.start();
        t1.join();
        t2.join();
        System.out.println("Flag: " + vd.flag);
    }
}