Artifacts appear with the rectangular tool in local processing

By using the local module "colour/tonality correction" with an area mask (rectangular tool) to make a black border around the photo (by inverting the mask so that it goes around the image), artefacts (black pixels) appear which overflow the frame.
This phenomenon does not occur with the polygonal tool (but it is less convenient to keep the horizontals and verticals).

These artefacts are detrimental for paper prints

thanks