Digital Light & Color

If the Mask transformation is inserted upstream of another transformation that has a Mask attached to it, the mask images are not available to the Amount/Mask button of that transformation, unless the original mask attached to it is Reset.

But, if any new transformation are inserted below the Mask transformation, the Mask images from the Mask upstream are available.

When you specify a mask from another transformation, it becomes the starting point for a new mask which you can then edit further if you wish. This is why you can only select such a mask if the current mask is empty.

Oh, it makes sense...