Okay, good to know how this is supposed to work at any rate. I can try to find a solution with ContentSizeFitter.
Can you double check that the UIMasked shader is still working in this update? I created a new game object in my canvas, gave it a Mask component, then created a child and gave it a SuperTextMesh with the UIMasked material. The text doesn't appear at all, whether it's inside or outside the mask. I can only see it if I change the material to Default or UIDefault.