Enderman AI has a lot of issues that seem pretty clear when they're aggressive. It's especially ridiculously obvious if you're farming endermen from underneath a 2-high ceiling. They constantly teleport to arbitrary places instead of trying to attack you and often forget about the player entirely, only to suddenly remember that they're supposed to be aggressive 30 seconds later. A lot of times they forget to do anything and just stand still for several seconds while you attack them. Their AI is so bad that even though they're supposed to be one of the strongest hostile mobs in the game, they're not very good at killing you. They hit you once or twice and then teleport somewhere else for a decent chunk of time, then teleport back and wait a few seconds before attacking you for some reason. Their pathfinding algorithm also doesn't seem to be fully aware that they have the ability to teleport, since I've noticed that it's very rare for them to teleport across gaps of more than a few blocks, which is especially obvious in the nether.

I tried to see if anyone else had reported this before and all I could find were some very old reports from 2014 saying that 1.8 broke their AI, so I'm assuming that the changes from 1.8 are, in some way, the root cause of this issue. However, those issues were all closed as duplicates of MC-47437. This isn't accurate, though, since that bug report only applies to them not teleporting away from hazardous blocks, not all of the other issues they have. The fix for MC-47437 is also partially negated by the fact that their AI sometimes randomly freezes as mentioned above, so they often stay in lava or water for several seconds before teleporting away.

This issue can be replicated fairly easy just by spawning some endermen and trying to fight them in survival mode (make sure you have armor/resistance to survive attacks). After a couple attacks, the issues with their AI become very obvious.