Opacity Mask Thumb displayed in color, not in grayscale
Please checkout the current Opcity Mask viewing. When you hold down the Option/Alt key to review the mask you see the objects or image in color. It should be like it used to be before in Gray scale... (black Conceals, White Reveals)
Why did you change it? In the imge I enclose you can see the dog in color..should be BW once it is in the mask area.
Chana Messer commented