Hi, I'm seeing a similar issue in 1.10.2 and 16w39a in one of my worlds, and trying to figure out if it's related or if I should file a new issue. Here are some screenshots: http://imgur.com/a/OOUVA. Could someone take a quick look and let me know?
Did some more testing. I think this happens if you start in creative, summon, then switch. I left the game in survival mode, quit, and rejoined the world, and tried again. Enderman aggro'd as expected.
I'm also seeing this. /summoned a few enderman, switched to survival using /gamemode survival, stared at one of them. Did not aggro. Eventually an iron golem came and killed it.
@Pokechu22 Yep, that's why I included chunk borders in most of the pics. Anyway, thanks for confirming, won't file a dupe 🙂