Mouse move events in QGraphisScene ignore the middle mouse button

Take a QGraphicsScene connected to a QGraphicsView with activated mouse tracking (graphicsView->setMouseTracking(true)). Logging mouse events in QGraphicsScene::event(QEvent *event) while using the middle mouse button shows the following:

Press the middle mouse button.

• EXPECTED: QGraphicsScene::event is called immediately with an event of type QEvent::GraphicsSceneMousePress

• ACTUAL: QGraphicsScene::event will be called right before releasing the button (see below).

Move the mouse while keeping the middle mouse button pressed.

• EXPECTED: The caused mouse events should have QGraphicsSceneMouseEvent::buttons() == Qt::MiddleButton

• ACTUAL: The events have QGraphicsSceneMouseEvent::buttons() == Qt::NoButton.

Release the middle mouse button.

• QGraphicsScene::event is called twice: First with an event of type QEvent::GraphicsSceneMousePress (which I expected earlier, see above), then with an event of type QEvent::GraphicsSceneMouseRelease (as expected)

For left and right buttons everything works as expected.

Minimal code for logging this kind of events is attached.