Endermen keep disappearing after provocation

I build a mob farm near build limit in the plains biome (such that mobs are forced to spawn there) , it consists of a single cobblestone floor and a half slab ceiling 3 blocks above the floor, such that endermen are able to spawn... however, more than half the time when I look at an enderman, they take a few steps towards me, then disappear, with NO indication of where they went. I check the roof and they're NEVER there, I check the ladder up, nothing... there's NOWHERE they could've gone... unless they SOMEHOW teleported the more than 100 block distance to the ground FAR below...