Bitmap brushes i made a while back for pixelart are buggy: some don't paint, some are wrong.
It looks like if pixel brush had odd size it would work incorrectly.
You can see 1px don't even register and 21x21 brush that is diamond-shaped checkerboard paints a blob instead of checkerboard.
Brushes file attached. Using pixel tool, obviously.
Pixel art.afbrushes