Reproduction steps:

1. Open "MaskIgnoresMaterialChanges.zip" project
2. Open "Repro" Scene
3. Expand "Canvas" in the Hierarchy window
4. Click on "GOOD" and drag "Image2" from the Project window to "Overlay (Script)" -> "Overlay"
5. Click on "Bad" and drag "Image2" from the Project window to "Overlay (Script)" -> "Overlay"

Expected Result: Both of the Images update Instantly upon dragging the "Imagine2"
Actual Result: The "GOOD" Object updates instantly, but the "BAD" one doesn't

Reproduced with: 2017.3.0a1, 2017.2.0b4, 2017.1.0p1, 5.6.2p4, 5.5.4p2