ShapeSubstract does not honor bind changes in a and b

insert ShapeSubtract {
            fill: Color.color(0,0,0,0.8)
            blocksMouse: true
            translateY: bind Main.navheight
            a: Rectangle {
                    width: bind layer.scene.width
                    height: bind layer.scene.height - Main.navheight - Main.menuheight
               }
            b: bind Rectangle {
               x: 20
               arcWidth: 15
               arcHeight: 15
               y: 34
               height: bind ci.height - 30
               width: bind ci.width
            }
        } into content;

The shape does not change if any of the bound attributes in a or b change.