filters picking up pixels from outside of the selected area ?

[Herbie22012266fjw7]

Is it me, and I don't remember how Photoshop works, or is this new in version 25.12 Mac? 

I wanted to blur the background and not the person in the foreground, but the blur filter is picking up on pixels OUTSIDE of the seleciton area (no Feathering! Feathering=0). I've done this thousands of times and do not remember the foreground outside of the slection area being picked up. I tired it on a basic image, ans you can see the gap between the selection area and the black field, and how the black field pixels are being picked up by the blur filter. This is driving me nuts! 

[Lumigraphics]

Double-check the selection, and your selection might be anti-aliased.