Button does not reflect changes to GradientStop.color

Consider the following snippet. Changing the value of the variables, buttongradientTop and buttongradientBottom, should result in the gradient colors of the button changing. This is the behavior of a Qt5 Rectangle element, etc. The color property of the GradientStop can be changed, but the color of the button does not reflect this change.

Button {
...
gradient: Gradient {
      GradientStop { position: 0.0; color: buttongradientTop }
      GradientStop { position: 1.0; color: buttongradientBottom }
   }
}