How to reproduce:
1.Create a Superflat world with the overworld preset;
2.Find a village;
3.You will see Grass spawning on village's gravel track.

Note: Found Grass inside small houses of villages too (dirt floor), think this should not happen.
Note2: Made a quickly search with the keywords "gravel" and "grass" to see if someone else hasn't posted it yet and don't. Sorry if I still duplicated it.