Grass blocks placed at certain coordinates in old worlds have a darker color on their sides than on the top. See the attached screenshots. The affected blocks include those at certain nonadjacent pairs of X and Z coordinates. The Y coordinate doesn't seem to matter, There is no obvious pattern to the affected X/Z coordinate pairs, except that I haven't seen any horizontally adjacent ones, and it doesn't appear to affect blocks over a very large area. The darker color resembles the grass color in a swamp.

The screenshots were taken from the attached world MCPE42418 demo.mcworld.