To reproduce:
1. Open the "Scene" scene in the user attached project
2. Select the "Material" from the Assets folder
3. Change the value of the "Invert" slider in the Inspector
4. In the Scene view, observe that it only affects the left image (whose parent Game Object does not have a UI Mask component attached)

Desired outcome: Child Game Object of a parent that uses a UI Mask Component should reflect changes made to the material.
Actual result: Using a UI Mask Component causes material changes not to be updated on child elements.

Note: The material has to be reimported for the changes to be applied.

Reproduced with 5.5.4p2, 5.6.3f1, 2017.1.0a1, 2017.1.0p3, 2017.2.0a1, 2017.2.0b6, 2017.3.0a3

Update: The issue is by design since materials for items under a mask component are copied. Thus changing a material via inspector will not affect the material used for rendering. Any effects need to be accessed via MaterialForRendering and thus can't be batch applied.