I know this is an old post, but I came across it searching for this problem. Tiled background shows the top tile edge as a thin line and on slopes like hills.
For me it was caused by using the eraser to rub out on the top to make curves and it showed the tile edge line as it scrolled. When I drew the curves with the brush and coloured in and did not use the eraser it showed perfect. A bug??
Edit update, it was because I filled I the whole tile with different colours that fixed it. If you leave the top part blank transparent that caused my issue. Filling the whole tile is working ok.