Endermen spawn in space 2 blocks high

I have a rail system two blocks high in The End, the roof is a half slab. Endermen are spawning in the space two blocks high. Their heads stick out of the half-slab, they are stuck there and cannot move. If attacked they slide upward through the half-slab.

Screenshot #1: enderman stuck in 2 block high space
Screenshot #2: same enderman's head sticking out of the half-slab

Edit #1: this issue did not occur in version 1.1.5
Edit #2: issue still exists in version 1.2.2
Edit #3: issue still exists in version 1.2.3