Strange error in photoshop

When making a complex alpha mask, I usually create the image in black and white first normally, and then copy and paste it into the alpha channel. However today I am creating 2 alpha masks for 2 versions of the same image. The second alpha mask must be mirrored in the Y axis. The first alpha mask on the first image works fine. However, when I mirrored the black and white image about the Y axis, and pasted the result into the alpha channel of the second image, it went all wrong. It looks as if it loses its "tolerance" as in only the purely white areas of the mask are now white, and everything else is purely black, there are no greys inbetween. So it seems that if you are using an image you're making as an alpha mask, no translations/ transformations can be done to that image, otherwise it won't work as a mask, weird! Has anyone else had this problem??