Structures are not recognized by the game after updating

If you have a map created in 1.6.2, and had a witch hut there, when migrating to the snapshot, witches will not spawn anymore on the hut, instead, normal monsters will spawn

What I expected to happen:

A witch to spawn in a witch hut generated in minecraft 1.6.2

What actually happened:

Witches did not spawn, normal monsters spawned.

Steps to reproduce:

Find a witch hut in version 1.6.2
See how they spawn
Update to the snapshot 13w36a
No witch will spawn, instead, normal monsters will.