Why does calling .join() on a Python multiprocessing child process attempt to close the IDLE shell?

The following python script:

import multiprocessing as mp

def f():
    return None

def main():
    print('create')
    p = mp.Process(target=f)
    print('start')
    p.start()
    print('join')
    p.join()
    print('done')

if __name__ == '__main__':
    main()

Produces the following output in Python IDLE:

Python 3.7.6 (default, Jan 30 2020, 09:44:41) 
[GCC 9.2.1 20190827 (Red Hat 9.2.1-1)] on linux
Type "help", "copyright", "credits" or "license()" for more information.
>>> 
===================== RESTART: /home/user/Desktop/test.py =====================
create
start
join

Followed by an (unexpected!) GUI prompt "Your program is still running! Do you want to kill it?"

Any clue what's going on here? I know printing from child processes is complicated in IDLE, but this seems unrelated.

Note that running the same code from the terminal gives "normal" output, and exits normally:

[user@localhost Desktop]$ python test.py 
create
start
join
done
[user@localhost Desktop]$ python --version
Python 3.7.6
[user@localhost Desktop]$ 

At a suggestion from a colleague, I tried adding mp.set_start_method('spawn') to the if __name__ == '__main__': block:

import multiprocessing as mp

def f():
    return None

def main():
    mp.set_start_method('spawn')
    print('create')
    p = mp.Process(target=f)
    print('start')
    p.start()
    print('join')
    p.join()
    print('done')

if __name__ == '__main__':
    main()

Which causes IDLE to behave how I hoped:

Python 3.7.6 (default, Jan 30 2020, 09:44:41) 
[GCC 9.2.1 20190827 (Red Hat 9.2.1-1)] on linux
Type "help", "copyright", "credits" or "license()" for more information.
>>> 
===================== RESTART: /home/user/Desktop/test.py =====================
create
start
join
done
>>>