When using In Control to add new mob spawns to The End, and a mod called V10verlap to connect the overworld to The End via the sky (reaching a height of 256 in the overworld will transport the player to a height of ~1 in The End, and moving back down will teleport the player back to the overworld), there's a glitch where some mobs (such as slimes) you've set to spawn in The End will briefly spawn in massive numbers in the overworld right after you move from The End to the overworld.

Um, that's weird but perhaps you should post it to the V10verlap issue tracker as clearly it is doing something weird. I don't think there is much that I can do to fix this on my side