In my superflat world a little distance out from my spawn chunks, squares of what appears to be concrete powder were on the ground instead of grass and when there was a block update on them they turned back into grass.

In my superflat world squares of concrete powder appeared a little distance outside of my spawn chunks that looked like the attached images and when I placed a block down on them or let water flow over them they disappeared and turned back into grass ( I promise I did not build these myself)