How to reproduce:
1. Open the attached project
2. Ope Masking Repro scene
3. The C. (bottom) example contains two UI.Mask where the mask on the large yellow button is disabled and the mask component on the small green button is enabled
4. As you can see, the masking solution is incorrect since text is seen out of green button with the mask component
Expected: text should be masked just like in the B. (middle) example

Not reproducible: 5.1.4f1