This, like many other bugs, has been occuring since snapshot 14w06a. Endermen will not teleport away to dodge melee attacks at all.

To Reproduce

1. Find/Spawn an enderman
2. Hit it to make it attack you
3. Keep hitting it until it is dead, aim at the torso and head areas
4. Notice how the enderman attempts to teleport (it disappears and comes right back) as you hit it

There is a 100% chance that the enderman will NOT teleport.