Your lightmap resolution is too low, which means that pixels will overlap.
also, there is no automatic method that will produce as good of results as doing lightmap UV’s by hand. In your case, it separated things far too much and you have a bunch of really small islands in the UV’s. Because of that, you would need a very high resolution lightmap. By doing it by hand, you reduce the amount of empty space and you can use a lower resolution lightmap since the small polygons would be attached to bigger polygons.