The mailman command should set effective user:group if run as root.

People tend to run mailman commands as root. This causes various permission issues. For example, running mailman import21 as root can create template files owned by root that can't be read by the Mailman user. Also, running mailman start as root will work, but can result in various files owned by root that can't be read by the Mailman user when mailman start is subsequently run as the Mailman user.

I think at a minimum, the mailman command should detect that it is running as root and if so, set the effective user:group to the owner:group of its file.