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.

I used /gamerule doMobSpawning true but it didn’t help.

Update: I made 2 almost-identical hollow boxes of width 1 (boxes whose walls are 1 blocks-wide) in area where I did see hostile mobs spawn naturally. The light levels inside of them were 0 – hostile mobs spawn at 7 or less to my understanding.
One of them I made with WorldEdit with //replace and various //expands and //contracts, and one of them I made manually by placing blocks.
The one I made with WorldEdit didn’t have mobs spawn inside, and the one I built manually did.

So I’m pretty sure it has to do with WorldEdit causing this.