No matter what elevation, condition, or difficulty, Endermen will not attack Endermites. I've tried this both on a server, and a single player world, and it happens on both. I can't confirm if this bug is on any other version, but this was not on the list of changes from what I am aware of.
MC-117495
Endermen don't attack Endermites.
Resolved
Invalid
0
The endermite HAS to be spawned via an ender pearl, not via spawn egg or normal /summon.