It reproduces when UI Object with a "Rect Mask 2D" has a child object (which is not the first child in hierarchy) which also has a Child with Image(script) and Mask(script) components. Also, there must be another UI element on the Canvas, so that object with the "Rect Mask 2D" is not the most bottom child of Canvas.

To reproduce:
1. Open "Test" scene, from "SimplerUiProject.zip"
2. Open Game (or Scene) window and inspect, that RectMask2d is working correctly.
3. In Hierarchy find "Canvas -> RectMask -> Parent_of_Mask" and enable "Parent_of_Mask" object.

Expected result: Enabling this object does not change the way RectMask2d works.
Actual result: RectMask2d is no longer working.

Reproducible: 2017.2.0b6, 2017.2.0f3, 2017.3.0b4, 2018.1.0a1
Not Reproducible: 2017.2.0b5

Workaround:
Make sure Object with RectMask2D is the bottom child in Canvas.