Rabbits falling through 1 layerd block when jumping underneed fence

I was trying to lead rabbits (with carrot) from one shelter to the other when i saw that rabbit were falling though 1 layerd block (with redstone torch underneed). The shelters were 2 fences high and the gates were 1 high with a fence on top of it. (I think only adults)

What I expected to happen was...:

I expecterd that i could lead rabbits without them falling trough blocks

What actually happened was...:

The rabbits fell trough the blocks

Steps to Reproduce:

1. Make an one thick layer platform one above the ground.
2 Make a fence on the platform so the rabbits won't jump of.
3 Make somewere in the middle a fence overhang (one above the platform from side to side.
4 Place rabbits inside of the fences on the platform
5 lead the (adult) rabbits from one side to the other with carrot (so they jump not walk)
6 See how some of them disappear (under the fence) in the ground a walk freely.

EXTRA

With a more layered ground the rabbits are also forced in to the ground underneed the fences