I set up a simple enderman farm right outside the portal to the outer ring of the end. I took wolfs, signs, and lava hoping to kill them quickly as they pressed against the square of blocks too low for them. It works moderately well, but I've noticed that some of the endermen lose interest after a while and go back to their idle behavior.
These endermen will not become hostile no matter what happens. Looking at them or attacking them will not trigger them, nor can other mobs (i.e. wolfs) get a reaction. However, the affected endermen will still teleport around the player and avoid attacks as if it were still in a hostile state.
Because of this, I've thought of naming one and keeping it as a pet, but I believe that would be a waste of a name tag as soon as I left that area of the end, so I'd rather have this bug fixed.
(Note that the third image showing me attacking the enderman in question is not opening it's mouth to suggest it is in a hostile mode.)