Chickens (And small mobs) cannot path when standing in/next to fences

I've noticed since the latest snapshots that small animals, when they are pressed up against say, a cobblestone wall, they won't ever move away from it, as if they think they're stuck inside of it with nowhere to 'path out of the block', even if they aren't actually stuck inside said block.

I've noticed this happening in 12w49a as well as the version tagged above. (This could also have been happening in 1.4.5, but I wasn't paying much attention to it at that point)

The screenshot below is just showing the chickens all crowded up in the walls, and have been like that... for in game weeks.

Edit: It seems to happen most frequently around the corners of wall blocks and fences.