Steps to reproduce:
1. Download and open attached project "RectMask2D.zip"
2. Open and play scene "bug"
3. Select "ChildCanvas" object in the hierarchy and change "Left" property value in "Rect Transform" component of the selected object
4. Actual result: Notice that rect mask 2d component from lower object moves independently from parent object
Expected result: Everything moves at the same time and speed

Tested and reproduced on:
Windows 10
Unity 5.2.4p1, 5.3.1p2, 5.4.0b1

NOT reproducible on:
Unity 5.1.4f1