I've been stuck on this for four weeks.
Here is another different copy of a systemctl script. What if you used this, modifying each value to match your environment. Change the paths. But don't add anything more like "RestartSec". Leave that out.
Hopefully it works, but if not, another random idea, move the PIDFile to /tmp, or omit it.
[Unit] Description=Mailman 3 Core service After=network.target Documentation=https://mailman.readthedocs.io/ ConditionPathExists=/etc/mailman3/mailman.cfg
[Service] ExecStart=/opt/mailman3/bin/mailman -C /etc/mailman3/mailman.cfg start ExecReload=/opt/mailman3/bin/mailman -C /etc/mailman3/mailman.cfg restart ExecStop=/opt/mailman3/bin/mailman -C /etc/mailman3/mailman.cfg stop Type=forking PIDFile=/var/lib/mailman3/master.pid SyslogIdentifier=mailman3 User=list Group=list
[Install] WantedBy=multi-user.target