Shape event using Xfixes region

[Buola Buola]

I am using an XserverRegion and the Xfixes functions to change the
shape of a window, and I notice that I don't receive any ShapeNotify
event, while I do receive it when I change shape with any of the
funcions of the XShape extension. Is this possible or I have to check
something again?
If this is true, is it a bug or some design decision that I can't understand?
thanks in advance,
    xavi