Grouped dragging in main window is buggy

Dragging dock widgets with grouped dragging feature is buggy. Playing around with widgets using examples ends with missing buttons, invisible widgets.

In my application, which code unfortunatelly I cannot attach, after few drags to undocked window strange empty rectangle is shown in the main window.

On attached screenshots (1 and 2) there are windows with missing buttons.

On 3 there's a checkmark next to the blue widget, but it actually isn't visible anywhere on my screen.

On 4 there's a strange gray rectangle on the left of the main window. After few drags, everything crashes.