Is this a bug

That's a valid join date. Why do you ask? Is the date incorrect for that user? A join date of 1999 would obviously be from an imported user account since XenForo didn't exist back then.
 
So the problem is that you can't explain a join date of 1999?

Did you import from another forum application? Any addons that manipulate the join date or user records?
 
I never started a user group

Check the other two things as well. As I said...

User titles can come from usergroups and the ladder of trophy titles, or from custom titles defined in user profiles.

And what about the date ?

I offered two guesses. Are those guesses not correct?

Another possibility is that your server clock is wrong... or was wrong when that user registered.

XenForo has no utility to edit the join dates of users.
 
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