I cannot see a particular User in the user selector
Nor mention him in a comment.
I'm site administrator, and I can locate this user in the users list. He seems to be in the right status.
This may not be you - In the last 2 days I have had 2 users 'disappear' in confluence (though I realize this is jira you're referring to) - support pointed me to this bug:
If you submit a real support ticket they can try to fix individual users.
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