There is a userid that is not defined to ACF2 that shows up on a LIST USERS command and it created a userid on the WebViewer server as well.
OM Web Viewer 12.1
Product working as designed
If a user with valid mainframe credentials logs in to Web Viewer, but does not have access to any role, that user is logged in using the Default User Role. If the user has not been previously defined, that user has a user object defined automatically, and placed in the Default User Role. For more information, Managing User Authentication.