I'm using the latest version of Multi Mob, with the config reset, and no manual edits.
[Forge 1.12.2-14.23.5.2808]

Zombies start to spawn as soon as I enter the Nether.
They infinitely spawn until they hit the mob limit, it seems.
It doesn't happen in the Overworld or the End. I haven't tested any mod-added dimensions, though.

I'm using a lot of mods, but it definitely stops happening when I remove Multi Mob.
Mod List

Edit: I tested with Multi Mob enabled and Primitive Mobs disabled and it still happens.

Wait with a config reset? That should be impossible. Are you sure? (Fully delete the multimob folder to reset properly)

Ugh... Looks like it's a mod conflict.
I tested with just Multi Mob installed and the problem doesn't happen.
I have no idea what could be conflicting, but I'll keep looking.

Let me know if you find anything. Maybe I can solve it.

...The bug stopped happening, and I have no idea why.
I thought I was onto something when disabling NetherEx finally made the zombies stop spawning.
But couldn't reproduce the bug at all in a clean test instance with just LibEx, NetherEx, and Multi Mob.
I put NetherEx back in my pack, along with Multi Mob and all the other mods... The bug stayed gone.
My pack is basically identical to how it was when I started this issue, except now the bug is gone.
I've tried deleting the NetherEx and Multi Mob configs to reset them, nothing will reactivate this bug.
I even restored a backup of the original configs, nothing.
I don't know what to say. Minecraft and Forge are very mysterious.

I did manage to take a screenshot of the bug while it was happening.

Glorious... maybe the config never really properly reset or something, maybe it was bugged out. Anyway good to see this being "solved". If anyone else has this issue; feel free to re-open this.

I got that also, sadly it seems there is no easy way to fix, so i'll try flushing some config too

I think I found something: in config/NetherEx/Biomes/[mods]/[biomes].json, there is a LOT of entry for "creatureType": "multimob_XXXX", that shouldn't be there. Might have to reset them all, and reenter my modification

Had this issue as well recently. Possibly due to an older MultiMob version. removing the minecraft:zombie entries from NetherEx fixed it for me.

Yeah, I reset my NetherEx config altogether after updateing Multimobs, fixed the problem