he @gsmith
usernames where not unique in Graylog, that is the reason that internal communication has changed to use the UUID of the user.
As this is used for authentication in the connection to Graylog itself the logging does not know the username anymore and has only the UUID available to write that into a log.
The only option would be to use the audit log that is available as part of Graylog Enterprise. The other option you build a lookup table that translate the uuid to a username and use that for enrichment.