Adult Horse/Donkey Pathfinding Issue: 1 block elevations treated as 'non-traversable' terrain in certain configurations

Horses wont jump out of/over 1 block high anything but can use stairs ok! First occurred in snapshot 15w40a and has been there ever since!

What I expected to happen was...:
Horse to be like normal

What actually happened was...:
Horses getting deeper and deeper in my world because once they jump down they don't jump back up! lol

Steps to Reproduce:
1. Make new world in 15w40a or newer and find a horse.
2. Make a hole 1 block deep by whatever width/length
3. push/spawn a horse in the hole and he's stuck. Also if you look around you'll notice all horses getting progressively lower and lower in the world and also because of this all the cave entrances are filled with horses lol!

Issue started appearing in 15w40a.