Animating masks scale via script.

Hi.

Since all masks in animate are discarted and recreated upon publishing, the anchor point is suposed to be in the center of an mask. But when i try to offset the center by adding or decreasing half of its height/width, the mask still does not scale from top. It still moves a bit as if the anchor point is still somwhere near center. How do we change that? 

Changing Regx amd regy does some random things to position.