iOS - Subset of active touches cancelled by UIKit - crash on assert

Sometimes when I use MultiPointTouchArea in QML, zoom over it with two fingers on iPad and then (without releasing two fingers) I add three more fingers and swipe them together (to minimize application), i get crash with message "Subset of active touches cancelled by UIKit".

I've tried to comprehend why there's:

if (!touches) {
m_activeTouches.clear();
} else {
for (UITouch *touch in touches)
m_activeTouches.remove(touch);

Q_ASSERT_X(m_activeTouches.isEmpty(), Q_FUNC_INFO,
"Subset of active touches cancelled byUIKit");
}

but I don't get it. When I replace it with just

m_activeTouches.clear()

everything works fine and there are no problems with touch handling.

What do you think about it? Why removing only some touches when we want to end up with empty m_activeTouches anyway?