No/rarly enderman spawns in the end!

I converted a world from a spigot server with the version 1.13. I copied all files from world_nether and world_end into the world folder bu I didn't replaced the files. Now everything works fine exept the enderman spawning in the end dim. It also just doesn't work on the main island. When you teleport to f.e. 0 100 1000. the endermans spawning behaiviour ist normal. Sometimes some enderman are spawning but this takes a while.