Villager AI (farming) broken

Farmers (and other brown-coated villagers) who were happily farming in 15w40 now stand around staring at ripe crops. They very occasionally remember how to break them, and then usually leave the drops on the ground to despawn and fail to replant.

The only time they seem to successfully do anything is when they "switch tasks" between farming and breeding attempts, farming and wandering, or farming and running for home at sunset. After one successful operation they get stuck again.

EDIT: Villagers who are pushed into the farm block can harvest/re-plant, so it appears to be an issue with the distance for walking to/from a crop and the distance for working on a crop/farmland block not matching up anymore.