Not sure if this is a bug or just a characteristic, when using a object such as a box and the alpha is set to zero then set this object as a mask for another object or image and have this object/image fly in passing over the mask the second object/image is also invisible, but if the the mask object has the alpha set 1 or higher the second object/image is visible when animating in over the mask object.
I would be comfortable with setting the alpha to 1 but for the fact that the mask object is slightly visible. Is it possible to have a transparent object as a mask? Hopefully this is just a glitch in the Matrix :-)