I am having the same issue. I am patching my app and noticed this new buggy behavior.
I made a test app that features three buttons that reveal a particular icon each made through newImage/newSprite masked with a unique mask texture. When the icons are revealed in any order using the buttons, it works correctly. But when the icons are contained in a separate group for each, where the group is masked, it gets bugged. Oddly, revealing sequentially does not behave correctly, but revealing in inverse order of when the icons were created works correctly.
It is easiest to see what I am talking about through the app. Click the 7 icons from left to right, then restart and click from right to left.