To reproduce:
1. Open my attached project, scene "scene"
2. In Hierarchy window, select Button
3. Try Applying any of three pictures to button's Target Graphic field
4. Change Button's Normal Color (or alpha)
5. Apply different Image to Target Graphic field

Expected: old Image is no longer affected by Button's Normal Color
Actual: old Image is still affected by Button's Normal Color

Reproduced in: 5.6.4p4, 2017.1.2p4, 2017.2.1p1, 2017.3.0p1, 2018.1.0b1

Note: undo doesn't fix the issue if new Graphics was applied - in order to return transparent image to opaque state, it has to be applied to button again