I've been seeing this too, but I think the cause is slightly mis-stated above. My normal users are all above UID_MIN, but I was still seeing the problem. However, I found that I had a bunch of system accounts above UID_MIN (qmaild, qmails, and so on), which were also appearing on the login screen. I'm not running qmail on this system any more, so I deleted the accounts, and my name is now showing up where "Invalid UTF-8" was before.
I've been seeing this too, but I think the cause is slightly mis-stated above. My normal users are all above UID_MIN, but I was still seeing the problem. However, I found that I had a bunch of system accounts above UID_MIN (qmaild, qmails, and so on), which were also appearing on the login screen. I'm not running qmail on this system any more, so I deleted the accounts, and my name is now showing up where "Invalid UTF-8" was before.