When you activate the brush, move the cursor to somewhere on the map, and then use arrow keys to move the map (right key and up key), the brush will leave something like shadow on the place it passes, as the image attached showed. (Windows 10, 64-bit)
Known, long-standing issue with calculating the repaint region. This can happen any time while showing a preview or oversized tiles while scrolling the map view, it’s just particularly easy to trigger when panning with the arrow keys.