All processes running on the same core

I found all processes on my machine to only run on a single core and their core affinity set to 0.
Here is a small python script which reproduces this for me:

import multiprocessing
import numpy as np


def do_a_lot_of_compute(a):
    for i in range(1000):
        a = a * np.random.randn(123789)
    return a


if __name__ == '__main__':
    with multiprocessing.Pool() as pool:
        pool.map(do_a_lot_of_compute, np.arange(10000))

htop looks like this:

And the core affinities are:

pid 15977's current affinity list: 0
pid 15978's current affinity list: 0
pid 15979's current affinity list: 0
pid 15980's current affinity list: 0
pid 15981's current affinity list: 0
pid 15982's current affinity list: 0
pid 15983's current affinity list: 0
pid 15984's current affinity list: 0
pid 15985's current affinity list: 0

So my question boils down to: Why are the core affinities all set to 0? There are no OMP or KMP environment variables set.

#SBATCH --cpus-per-task=8