Adult Horses won't jump

Adult horses won't jump onto higher elevation blocks. Young horses don't have this problem.

What I expected to happen was...:
Adult horses jump to higher elevation when forced to.

What actually happened was...:
They won't jump to higher elevation and get stuck in lower elevation areas even when it should be possible for them to jump.

Steps to Reproduce:
1. Trap an Adult horse in pit with one block high walls.
2. Try to lead it out.