When you call setScale on a scaleDecorator that's already in the display tree, the display does not update. You can't see the effect of the change until something else causes a repaint.
Looking at the code, isn't this an issue for decorators in general? I don't see any mechanism for decorators to notify the component (or components?) that they are attached to that they've changed. If you're not going to notify, then shouldn't decorators be immutable objects? (Of course, that would break a lot of code.)
FWIW, a demonstration:
<Window title="Scale change" maximized="true"
<BoxPane orientation="vertical" preferredWidth="300">
<Label text="This text should get twice as big when I push the button">
horizontalAlignment="left" verticalAlignment="top" />