This seems like a relatively new phenomena. When I have a threshold adjustment layer clipped to the image, the edges of the image show all sorts of gray pixels. I can't think of any reason this should be happening. Adding to the weirdness, if the adjustment layer is unclipped, then the image it rendered as expected - total black and white.
My PSD eventually has dozens of differing threshold needs, so the unsnapped option isn't doable. At a loss!