Searched and thought https://bugs.mojang.com/browse/MC-119 was the same problem. Apparently not.

https://bugs.mojang.com/browse/MC-104259 Seems similar and may be the same problem.

Problem is Villagers, Mobs, and yourself can fall into blocks when the farmed dirt block becomes regular dirt.

How to reproduce:
1. Creative mode:
2. Build glass wall around villager on vegetables.
3. Create hole for you slip in.
4. Harvest what ever plant is under mob.
Continue destroying/harvesting plant/weeds until no new seeds are laid by villager.
5. Villager should now be standing on manicured dirt.
6. Wait for manicured dirt to become regular dirt.
7. When manicured dirt -> regular dirt, mob falls into block.