[Minecraft 1.16 | Hostile Mobs not Spawning in An Area (Edited with WorldEdit)]

I'm playing v1.16 with Forge v36.1.0. I used WorldEdit on a previously-played world to edit a selection - I regenerated the blocks in it and change the biome vertically (made a 2D selection and //expanded with vert).

Now no hostile mobs spawn in that area.

I can confirm that mobs spawn outside of what I remember the selection is - both on the ground, below ground and above ground - but not inside that area.

It didn't happen before the editing with WorldEdit but I can't verify that because I was playing on Peaceful for almost all the time.

I tried contracted WorldEdit developers but they didn't know why that happened and thought this doesn't have to do with WorldEdit.

Can anyone please help me debug this? I don't even know where to start.

[Forge + Quark | Transfer Part of World/Full World from v1.16 to v1.12?]

Hi, all,

I have a world created in v1.16 with Forge and Quark.
Is it possible somehow to transfer a part of the world to v1.12? I want to start using EnderIO but unfortunately it's not available for versions above v1.12.