Where is the “white requirement” documented?
Is it because Graphics.ClearRectangle is normally used to clear or ?
A mask is a grayscale representation of the desired alpha channels. With the transparency support, the entire picture of the desired mask is, by default, transparent, which is then turned to fully opaque when used as a mask due to the framework changing the alpha value of the color. Adding the white fill sets that area to fully transparent when it is used as a mask.
I don’t know that it is documented anywhere, just speaking from extensive experience.
ClearRectangle on an Alpha channel picture will clear it to a fully transparent surface, which is unsuitable for a mask.