To reproduce the bug, start the application, put your mouse cursor on the QGraphicsView without touching the QPushButton. With key 0, 1 and 2 you can change your cursor type.
Now pass over the push button and get back to the QGraphicsView. When you change the cursor type, it has no visual feedback but the property "cursor" of the widget has changed.
With the key R you can reset the QGraphicsView viewport and it's works again, until you pass over the QWidget. The problem is present with a software or opengl viewport and only when there is a QWidget in the QGraphicsScene.

The attachment contain a basic .pro project which reproduce this problem.