It's not a subprocess bug, per se. It's due to creating the stdout/stderr worker threads from the __del__ finalizer while the interpreter is shutting down. Minimal reproducer, confirmed in both Linux and Windows:

    import threading

    class C:
        def __del__(self):
            t = threading.Thread(target=print, args=('spam',), daemon=True)
            t.start()

    c = C()
    #del c # uncomment to prevent hanging