Certain Mobs Escaped Pens made of fences during an update from 1.7 to 14w21b

On my 14w21b server, Me and My friends experienced mobs escaping their pens (made of fences) from the update. I think it might be because of the way Entity Hitboxs interact now, but we weren't sure. The Instance in which the problem occurred was with Chickens. It only happened once the update passed.