I have created simple scene for test in Unity 5.4.2f1 and added scroll rect with mask sprite.
Sprite settings are these:
In the Unity Editor and on all my android devices, such as Samsung Galaxy Tab Pro and Nexus 5X are working perfectly:
But on my HTC One (Android 5.0.2) it's working not correctly. Mask script is behaving like Rect Mask 2D script: