UI detach-event is not fired after 20 minutes

I've added a detach-listener to my UI like this:

this.addDetachListener(new DetachListener() {

            @Override
            public void detach(DetachEvent event) {
                
                System.out.println("DETACHED!");

            }

        });

As far as I know the console should show "DETACHED!" after three missed heartbeats. However, after 20 minutes (I've configured default heartbeat with 5 minutes, so 15 minutes should be the maximum), there was still no "DETACHED!" to read.

Worth to mention: I've configured a button to manually close the session like this:

UI.getCurrent().close();
                VaadinSession.getCurrent().getSession().invalidate();
                Page.getCurrent().setLocation("");

This fires the detach-event just fine. Yay.

Thanks

Nevermind ... just in the second I hit submit on this post, I received it -.- :D Thanks!

However, still a bit strange that it took about 25 minutes instead of 15.