Endermen teleport into the void.

When i use my enderman farm i cant help but notice the endermen teleporting into the void for no reason. I recorded it happening to make it clearer.:

https://www.youtube.com/watch?v=BpmisWSKpII&feature=youtu.be

if you want to recreate the scenario, my farm is identical to the one in this video:

https://www.youtube.com/watch?v=ckhe8ws