Blitzcrank pull stuns but doesn't pull

In bots, Blitzcrank pull stunned Nasus bot but did not pull him. This happened twice, both when he was next to Renekton bot. I will try to reproduce it in a custom.

Edit: Reproduced it against Renekton bot. It pulled him halfway, then he sliced and diced immediately after the stun wore off. It seems to only happen when 2 champs are next to each other, and even then only sometimes.