Since migration to the new forum software, there have been several posts about users showing the wrong username. In my experience, this can happen when migrating from one database to another where usernames are associated with an internal ID number, and the data migration into the new database matches up by that ID number rather than the username.
Example: Old database user has username Frank with ID number 123 and new database has ID number 123 as Alice. After the migration, user Frank shows up as Alice.
If this is the case, there should be a procedure available to re-sync the username data to get the correct usernames for everyone rather than fixing them one at a time as they are found.