I have recently noticed that one username is not showing up in the Created By or Modified By fields. That name will also be blank if used in the Person or Group field. When trying to troubleshoot it I found that that user is not showing up in the hidden
All People security group. I don't know if this is the reason for that problem or just another symptom. That issues is affecting only one site collection.
Any ideas how to fix it? User profile sync seems to be working OK.