I have noticed an unexpected behavior with Photoshop CC 2018 in how Layer Masks are calculated. To illustrate this issue (e.g. in RGB 16 bit/channel), consider a solid black background layer with a solid white layer on top of it. The white layer has a Layer Mask consisting of a uniform 50% Gray, i.e. #808080. Common sense says that the resulting color of this white mask, showing 50% through to a black mask, would also be a 50% gray (and this is confirmed by setting the opacity of the white layer to 50%). However, this is not what happens with the Layer Mask: the result is considerably darker than this, with a Hex value #696969. My question is why? What formula is Photoshop using to generate this, and is this desired behavior? What's going on here?