Closing as not a bug. This does not reproduce in the Tilemap object when seamless mode is on, only Tiled Background. Tiled Background uses wrapping on both axes (horizontal and vertical). Since you use Letterbox Scale and not Letterbox Integer Scale fullscreen mode (and for pixel-perfect display, Pixel Rounding and Point sampling), the display will use floating point scaling, and render objects at fractional pixel offsets. This means in some cases a fractional vertical offset in the Tiled Background object, which then wraps vertically and samples the bottom row of the image along the very top of the object rectangle. This is just how computer graphics works when you enable floating-point offsets; the workaround is to use letterbox integer scale, point sampling and pixel rounding, or low-quality fullscreen mode with point sampling.