Problems with opacity of 2D elements in Viewport

Consider following code:

import QtQuick 2.0
import Qt3D 1.0

Viewport {
    width: 500
    height: 500
    Rectangle {
        id: rect
        width: 50
        height: 50
        MouseArea {
            hoverEnabled: true;
            anchors.fill: rect;
            onEntered: rect.color="white";
            onExited: rect.color="grey"
        }
    }

    Rectangle {
        width: 400
        height: 400
        x: 50
        y: 50
        opacity: 0.1
    }
}

Now enter and exit the MouseAre several times.

The small rectangle has a MousArea with hover enabled and on every entry or exit, the color of this small rectangle is changed. This works without problems.

The bigger rectangle however has an opacity and every time the color of the small rectangle is changed, the whole 2D content is is overpainted, without clearing it before. This increases the opacity of the bigger rectangle everytime the color of the small rectangle is changed.