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I've seen a similar problem which was resolved by stopping the server, deleting the ARSystem\bundle-cache directory and restarting. It appears that some of the user/group information may be cached between runs and stored in this directory. There shouldn't be a problem with clearing the contents of this directory as the server will recreate it as required. If you want to play it safe move/rename rather than deleting.
I had 2 users with the same issue. After deleting the bundle-cache and restarting the ars service, I could update the login id of 2nd user. But the issue still remains for the 1st user (Mark Miller).
After restart, user is somehow created in user_cache table even though user does not exist in user form.
I've updated user with a differerent login id (mark.miller2) and the changed the loginid from mark.miller2 to mark.miller using Data Wizard. It worked successfully. I was expecting it to fail when updating user form but it did not.