Round corners squished even scaling with RC effector

If you scale X and Y differently the round corners will always distort. As long as both X and Y scale value is equal, the corners will keep their constant radius. 

You can adjust the Transform Rectangle/Anchor Point to change the center of the Transform Rectangle/Scale. It works the same way as Layer/Transform/Anchor Point and Layer/Transform/Scale.

If you want to keep the roundness perfect, animate the X and Y size value of the rectangle.

To animate the size of the rectangle and keep the bottom from moving add this simple expression to the Transform Rectangle/Anchor Point. It adjusts the anchor point to half the Y size value and keeps the bottom of the Parametric rectangle from moving.

y = content("Rectangle 1").content("Rectangle Path 1").size[1] / 2;
[0, y]

Here are all the modified properties of my shape layer:

The only thing causing your problem is workflow. You are not using the controls properly. 

The path is a parametric entity. It's center canmot be changed. The rest should easily be fixed by re-ordering and re-grouping your items, though it's impossible to say what actually may be going on since you haven't included the comp view. Could just as well be that you have messed up the comp pixel aspect ratio or the layer's global transform, in which case of course that would need to be rectified.

Mylenium