i don´t know the size of the texture in your example, but could it be possible, that the textures size is bigger than the polygons size, so that one tile fills out the whole polygon?
In my case the polygon is much bigger than the texture, so that the texture has to be repeated to fill the whole polygon. Between these texture tiles i have always an empty space. Could you please try the following code with the polygon larger than the texture (my texture is 50×50 px):
the texture tiles of the PolygonItem need to have a power of 2 size (16×16, 32×32, 64×64, …), because this is very common for textures. I will have a look if we can add support for any sizes of textures.
Until then, is it possible for you to scale your tiles to a POT size?