Yeah, definitely a bug. I'm surprised there aren't more people in here. This has been happening to me and my buddy as long as we've been playing (since release).

For instance, at the Ixali camp in Coerthas, you can pull a mob not even 15 yalms from where he was standing, be dead in the middle or even at the end of a fight with him, and all of a sudden he just runs back like nothing ever happened.

Extremely annoying.