Hi Zeno,
Yes, you found it! My tile width was at 96, but when I set the tile width to 64 the lines disappeared. If I set it to any larger width they reappear.
Personally, I think that 96 is the best compromise between appreciating the art and viewing a large enough region in the window, so I'll leave it at 96 even with the gridlines. But it would be nice if this were resolved in a future version.
Cheers,
Ben