Witches no longer spawn in huts

Game world created in 1.7.2. Witch hut trap created and completed in 1.7.4.
In snapshots 14w03b and versions prior to this, the witch hut just spawns witches at the expected optimizable amounts, with the very occasional spider and skeleton (<1%).
Starting from 14w04a, and snapshots since then up to most recent (14w05b), regular hostile mobs spawn in the trap, with witches at the default very rare amount.