Trying to understand Django select_for_update() with MySQL and Processes

I have a django application that is backed by a MySQL database. I have recently moved a section of code out of the request flow and put it into a Process. The code uses select_for_update() to lock affected rows in the DB but now I am occasionally seeing the Process updating a record while it should be locked in the main Thread. If I switch my Executor from a ProcessPoolExecutor to a ThreadPoolExecutor the locking works as expected. I thought that select_for_update() operated at the database level so it shouldn't make any difference whether code is in Threads, Processes, or even on another machine - what am I missing?

I've boiled my code down to a sample that exhibits the same behaviour:

from concurrent import futures
import logging
from time import sleep
from django.db import transaction
from myapp.main.models import CompoundBase

logger = logging.getLogger()
executor = futures.ProcessPoolExecutor()
# executor = futures.ThreadPoolExecutor()


def test() -> None:
    pk = setup()

    f1 = executor.submit(select_and_sleep, pk)
    f2 = executor.submit(sleep_and_update, pk)

    futures.wait([f1, f2])

def setup() -> int:
    cb = CompoundBase.objects.first()
    cb.corporate_id = 'foo'
    cb.save()

    return cb.pk

def select_and_sleep(pk: int) -> None:
    try:
        with transaction.atomic():
            cb = CompoundBase.objects.select_for_update().get(pk=pk)
            print('Locking')
            sleep(5)
            cb.corporate_id = 'baz'
            cb.save()
            print('Updated after sleep')
    except Exception:
        logger.exception('select_and_sleep')

def sleep_and_update(pk: int) -> None:
    try:
        sleep(2)
        print('Updating')
        with transaction.atomic():
            cb = CompoundBase.objects.select_for_update().get(pk=pk)
            cb.corporate_id = 'bar'
            cb.save()
            print('Updated without sleep')
    except Exception:
        logger.exception('sleep_and_update')

test()

When run as shown I get:

Locking
Updating
Updated without sleep
Updated after sleep

But if I change to the ThreadPoolExecutor I get:

Locking
Updating
Updated after sleep
Updated without sleep