This is intended behavior. Supposing a 320×240 background, it should work if you make the map’s width a multiple of 20 and the height a multiple of 15.
The rule is: “When a “not moving background” is “linked to scroll”, the image displayed when the hero is at coordinate 0,0 should always be the same.”
Because of this rule, if the map size(in pixels) is not a size evenly divisible by the width and height of the background image, you will see the background jump into place when you loop around.