I experience a problem with multiprocessing and print.

I tried to make a minimal working example, please see the attached file.

WITHOUT the offending print statement in the queue filler thread, everything works:
- pytest experiments/mp_problem.py
- pytest experiments/mp_problem.py -s
- python experiments/mp_problem.py

WITH the offending print statement, not so much:
- pytest experiments/mp_problem.py WORKS (Probably because pytest captures fd 1)
- pytest experiments/mp_problem.py -s FAILS eventually (after a couple of workers have been started)
- python experiments/mp_problem.py FAILS eventually (same).

WITH the offending print statement AND PYTHONUNBUFFERED=1, everything works again:
- pytest experiments/mp_problem.py
- pytest experiments/mp_problem.py -s
- python experiments/mp_problem.py

Environment:
Ubuntu 18.04.5 LTS
python 3.8.5 (hcff3b4d_1) on conda 4.8.3