Understanding `CmdStanModel` Behavior with `os.fork`

I've run into a somewhat odd (and likely edge-case) situation where CmdStanModel.sample behaves unexpectedly after a call to os.fork. I'm providing pseudocode now in the hopes that there is a fairly simple explanation, but happy to make a minimum working example if need be.

Say I run something structured like the following:

import os
from cmdstanpy import CmdStanModel

def run_job():
    # Compile the model
    mod = CmdStanModel(stanfile, cpp_options={"STAN_THREADS": True})
    
    # Run sampling on a detached child process
    pid = os.fork()
    if pid == 0:
        os.setsid()
        mod.sample(data = data, chains=4)
        os._exit()
    else:
        return pid

Even though my CPU has 32 cores, this will only run two chains at once--specifically, Chains 1 and 3 run, then Chains 2 and 4 run. If, however, I remove the cpp_options={"STAN_THREADS": True} kwarg, then everything behaves as expected: all chains sample in parallel.

Is there any immediate explanation for this discrepancy in behavior?

Start with the interaction between CmdStanModel.sample, cpp_options={"STAN_THREADS": True}, and the os.fork path shown in the report. Reproduce the two cases with a minimal working example, then compare chain scheduling after fork; done means the discrepancy is explained and any necessary behavior or documentation change is covered by a regression test.