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И что же вы проверили?

var shape:Shape = new Shape();
shape.height = 50;
shape.graphics.beginFill(0xFF0000);
shape.graphics.drawRect(0, 0, 30, 30);
shape.graphics.endFill();
 
addChild(shape);
 
trace(shape.height, shape.scaleY); //0, 0
trace(shape.width, shape.scaleX); //0, 0
//нет шейпа. Вполне естественно при размере 0
shape.height = 30;
shape.width = 30;
 
trace(shape.height, shape.scaleY); //30, 1
trace(shape.width, shape.scaleX); //30, 1
//упс появился

А так вообще не исчезает

var shape:Shape = new Shape();
shape.height = NaN;
shape.graphics.beginFill(0xFF0000);
shape.graphics.drawRect(0, 0, 30, 30);
shape.graphics.endFill();
 
addChild(shape);
 
trace(shape.height, shape.scaleY); //30, 1
trace(shape.width, shape.scaleX); //30, 1