CAG operations, like ShapeSubtract and ShapeIntersect, doesn't respect binding: they seem to be done at creation time and doesn't change afterward.
Simple demonstration:

var circle1 = Circle
{
  centerX: 100
  centerY: 100
  radius: 50
}
var ellipse1 = Ellipse
{
  centerX: 100
  centerY: 130
  radiusX: 30
  radiusY: bind tradius + 10
}

var circle2 = Circle
{
  centerX: 200
  centerY: 100
  radius: 50
}
var ellipse2 = Ellipse
{
  centerX: 200
  centerY: 130
  radiusX: 30
  radiusY: bind tradius + 10
}
var ellipse3 = Ellipse
{
  centerX: 300
  centerY: 130
  radiusX: 30
  radiusY: bind tradius + 10
  fill: Color.LIGHTBLUE
}

// ShapeSubstract doesn't follow binding
var shapeS = ShapeSubtract
{
  a: circle1
  b: ellipse1
  fill: Color.LIGHTGREEN
}
var shapeI = ShapeIntersect
{
  a: circle2
  b: ellipse2
  fill: Color.LIGHTGREEN
}

def BASE_SIZE = 300;
def MARGIN = BASE_SIZE / 30;
def RADIUS = BASE_SIZE / 2 - MARGIN * 4;
var a = 0;
Stage
{
  title: "ShapeXxx Test"
  visible: true
  scene: Scene
  {
    width: BASE_SIZE * 1.618
    height: BASE_SIZE
    fill: Color.web('#00BABE')
    content:
    [
      shapeS,
      shapeI,
      ellipse3
    ]
  }
}

var tradius;
var animation = Timeline
{
  keyFrames:
  [
    KeyFrame
    {
      time: 0s
      values: tradius => 10
    }
    KeyFrame
    {
      time: 10s
      values: tradius => 100
    }
  ]
};
animation.play();