I've just unsubscribed a member from a Wikimedia mailing list whose account (which contains his afaik never-publicly-disclosed real name and therefore won't publish here) has the í character. This resulted in a unsubscription email body as follows:
=?utf-8?q?[redacted]?= <$member_email> has been removed from $mailing_list.
I assume this is some sort of encoding bug on mailman3?