I created a new world with the latest snapshot, and started terraforming a world, when I noticed the dirt wouldnt fill with grass. I had the tickspeed set to 1000, I tried various other tickspeeds, but I had the same issue.

What I expected to happen was...:
The dirt blocks adjacent to grass blocks should be turning into grass blocks.

What actually happened was...:
The dirt blocks adjacent to grass blocks stayed as dirt blocks, no matter my tickspeed.

Steps to Reproduce:
1. Create a new world
2. Find a hill of grass
3. Destroy the grass blocks until some dirt is visible, and turn the tickspeed up to 1000 or some other large number.