Weird behaviour when using gevent+uwsgi+gevent.subprocess

uwsgi.ini

[uwsgi]
master=true
gevent=1000
wsgi=wsgi
http=127.0.0.1:9000

wsgi.py

import gevent.monkey
gevent.monkey.patch_all()

import threading
import time
from gevent import subprocess


def start_long_background_subprocess_in_thread():
    def start():
        # This works! You can try it by uncommenting.
        # time.sleep(1)

        # But this does not!
        subprocess.Popen(['sleep', '5'])

        # Even this works! But only when time.sleep finishes AFTER the request
        # was responded.
        # time.sleep(1)
        # subprocess.Popen(['sleep', '5'])

    threading.Thread(target=start).start()


def application(environ, start_response):
    # The request is not completely finished,
    # until the subprocess is over. Though uWSGI even writes the logline
    # that the request was finished quickly, and was not waiting for anything.
    # Example:
    # GET / => generated 2 bytes in 2 msecs
    start_long_background_subprocess_in_thread()
    start_response('200 OK', [('Content-Type', 'text/plain')])
    return ['ok']