Steps to reproduce:
1. Create a GameObject "Root" in a canvas
2. Add the component Image (or RawImage) and the component Mask to "Root"
3. Create a child "Child" and set it as a child of "Root"
4. Add the component Image (or RawImage) to "Child"
5. Observe that "Root" is masking "Child" correctly
6. Remove the component Image (or RawImage) from "Root"
7. Add a component RawImage (or Image) to "Root"

Expected result: "Root" should be still masking "Child".
Actual result: Child isn't masked anymore.

Reproduced in: 2017.4.25f1, 2018.3.12f1, 2019.1.0b10, 2019.2.0a10