Nice find! The (inherent) mask of the B&W adjustments layer is not fully white. This is not easy to spot, but clearly visible if you toggle between the 2 top layers, and focus on the "Pixel Alpha" symbol in channels panel. When you use channels panel to inspect alpha channel only (and create a greyscale layer), you can see that the mask is actually not fully white, but a bit smaller (1646x933) than the other layers / document size (1719x1040). The point that lets me assume there is