This issue began to occur in 1.8 where Endermen were updated to use the pathfinding AI.

After initially aggroing the player, Enderman dodging behavior is normal, it teleports away from the player when their crosshair is aimed at their chest in close range. But once it is eventually damaged by a player, this behavior ceases to function. Instead, all melee attacks after the first one will cause Endermen to appear to teleport, but fail to do so and effectively stay in the same spot. This gives a lag like appearance where the Enderman will fly across the screen but then quickly come back to where he was (Similar appearance to mobs that appear to glitch out of walls and fly back in)

To reproduce:
1. Look at an Enderman to aggro him
2. Manage to damage him once (This is what causes the bug. If you don't manage to damage him, he will successfully dodge until you do so.)
3. Aim at his chest area to try and trigger his dodging
4. The Enderman will fail to dodge attacks and will take every melee hit

Screenshot explanation:

1st: Initial damage to the enderman
2nd: Enderman attempts to teleport away
3rd: Right after the 2nd screenshot, enderman appears back where he was, letting me continue to melee him in the same spot.