The MailmanProvisioner assumes that the role returned for a call to

/addresses/<email>/memberships

will be one of member, owner, or moderator. The API may also return the role nonmember.

This can happen if the Mailman admin (or other privileged user) has used the Mailman API or UI to add an email address to a list as a nonmember, but that email address is also associated with a user that is managed by the plugin. 

This seems to be a common pattern for VOs–need to add somebody to a list so they get certain emails but later they enroll in the VO or otherwise are added to a managed list as a member.