Recently, the mob spawning algorithm was changed to no longer require air blocks for mobs to spawn in. As a result, mobs can now spawn in nether portals. Upon spawning, these mobs are transported to the nether (creating a short lag spike), where they will stay until a player arrives. If a player doesn't use their portal(s) for a while, they will be greeted with enough mobs to kill them once they do.
This mainly affects slimes as other mobs require a light level below that produced by nether portals.
To reproduce:
1. Create a flat world
2. Find a slime chunk
3. Place 16 16-wide max height portals on it (so the whole chunk is covered in portals)
4. Place half slabs on all other chunks and on top of the portals to prevent other mob spawns
5. Light the portals
6. Wait a few minutes
7. Go through one of the portals and observe the slimes that have appeared by the nether portal on the other side
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How about I explain. The mobs can only spawn in the nether portal when replicated, as you said. And so, they are transported and stored. This is intentional. Throwing items or pushing mobs into portals is intentional. Stored puts their position, NBT Tags and names into storage and puts it into play when entering the dimension, making mobs (or items) spawn in the nether when you arive. So, simply, The slimes spawn for one frame then go into the portal, get stored, and once you go in, they are unstored and put into play. By how long you stayed, a certain amount of slimes will be in. Bam. Simple fix - Don't cover everything in slabs. Also, This is the same thing on how it works, but a little different, with ender pearls. You throw a pearl into the air and go in a different dimension. It stores the position of the pearl and what direction it was moving towards at the time. This makes it so you can go to the nether, leave, and end up where the pearl would land.
The reason everything is covered in slabs is so it's easier to reproduce. I still don't understand why you think this isn't a bug.
It isn't a bug. It's a simple mechanic in Minecraft. Entities get stored when they enter portals at their position, so they start building up amounts. I basically explained what it is previously, but you can do this without slabs by pushing mobs in portals.
Confirmed for 1.12 - just happened to me. Large slime spawned in stronghold portal and immediately teleported to the nether (unexpected surprise to me going through). Checked chunk boundary and it splits my portal pretty much evenly. Considering I have been working on cleaning up end portal room, clearing caves, and digging around to see if there is any of the stronghold I missed since it had nothing pretty much nothing except end portal room and did not see one slime the entire time. not sure where the slime chunk is. Walkway from portal to end portal is 3x3.
Same in snapshot 19w13b , i build a nether portal pigman farm and when i go back to the nether , there is billion of mob in caused by the farm
confirmed for 1.16.1 still an issue.
for anyone who ends up in a slime chunk and spends alot of time in the area, the game can have problems loading the nether when you do finally go through. If killed when going through the slimes do despawn. This is a big problem for realms or servers that have a community area within a slime chunk and prefer both portals to be low down in the world for better linking and easier nether tunnelling
Not a glitch. This is intentional.