Comment 3 for bug 1832740

A workaround for your issue is to run (as root or the mailman user)

/usr/lib/mailman/bin/mailmanctl restart

instead of

systemctl restart mailman

systemctl restart is actually doing a stop followed by start as you observed whereas mailmanctl restart will just signal the qrunners to restart.

You might also be able to adjust your systemd mailman service script to do mailmanctl restart instead of stop/start.