To reproduce:

• Start an animation form ResizeEvent of QWidget (might also be QGraphicsItem/QGraphicsWidget/QGraphicsLayout etc.)
• Double click the title bar of the window to maximize. (Anim plays)
• Double click the title bar again to restore it back to original size. (Anim does not play)

Note it might play some frames - this happens if any other events are delivered to the window, like mouse-over, activate or other.

Attached is a sample, showing the problem.