I was trying to make an Enderman Farm when suddenly I noticed that if you are in between the End Main Island and End Cities Islands Endermans stop spawning and Overworld Hostile mobs start spawning (This only happens there).

-Ways to Recreate it (And the way I noticed):

-Create a Bridge in the coord y:1 and prolong it 200 blocks (any direction)

-Create a Platform 20x20 and get away 128 blocks, then, get back and see If any Mob Spawn that is not an Enderman (For this you may not have lights in the Platform)

As always I can't provide Screenshot, because it wouldn't verify that the mobs spawned naturally.