Actionscript 3 :: Canvas' VerticalScrollPosition Cannot Be Changed In Flex

I have a weird problem with verticalScrollPosition in Flex.I have a content Canvas and a wrapper Canvas. The content is large (5000px X 5000px), the wrapper is 800px X 800px.

public var wrapper:Canvas = new Canvas();
public var content:Canvas = new Canvas();
wrapper.addChild(content);
application.addChild(wrapper);

I would like to set the wrapper's scrollbar position dynamically anytime. I can do it by calling its properties:

wrapper.verticalScrollPosition = A;
wrapper.horizontalScrollPosition = B;

This is working fine. But! If I set a default scrollbar position when the Canvas is complete:

wrapper.addEventListener(FlexEvent.CREATION_COMPLETE, function(e:FlexEvent):void{
wrapper.verticalScrollPosition = DEFAULT_A;
wrapper.horizontalScrollPosition = DEFAULT_B;
});

I can't set the verticalScrollPosition anymore:

wrapper.verticalScrollPosition = C;
trace(wrapper.verticalScrollPosition); // Outputs: DEFAULT_A

So the problem only exist if I set a default position using 'FlexEvent.CREATION_COMPLETE'.