I haven't used shape tweens in a very long time. It seems like it's working differently than it used to.
I'm expecting it to maintain the number of points, maintain the point order, and tween each anchor point and handle position separately. Anime Studio works exactly the way I expected it to work (and believe it used to work). With CS5, I have to add shape hints just to sort-of maintain the point order. And if I ever add a keyframe between 2 existing keyframes, there's a gazillion new points to deal with. Is there any workaround?
This info helped me, when I got weird results:
Shape hints work best if you place them in counterclockwise order beginning at the top left corner of the shape.