It seems the Stamp Point node, which places the Sprites inside the Tile Pattern node, clips the sprites when it renders them, and so the Tile Pattern node is not seamless when applying variations to the grid. The attached image shows a 2D Transform offset.
All you need to do is play with uniform scale and you already see that the stamps are clipped. Play with variation also and it becomes more apparent. It seems to be simply not working as it should.
