I have a Scroll Rect with a Mask on a Canvas set to World Space, it looks fine in Editor but on my Nexus 5 the mask is not being applied. Used to work fine before 5.2 I think but I have made a few changes since last time I tested so I can't guarantee that the Unity update is what broke it.
"Use 32-bit Display Buffer" is checked, "Rendering path" is set to Forward and "Disable Depth and Stencil" is unchecked under Player Settings . The image is a Raw Image.
Any ideas?
Try re-testing with Unity 5.3, the UI system seems to have stabalised a bit and has also had it's masking components replaced with a new clipping framework.
Alternatively try the Soft Alpha UI Mask as part of the UI Extensions project (https://bitbucket.org/ddreaper/unity-ui-extensions) which is a shader based approach with alpha support.
Hope this helps