There are quite a few User 'NAME' exists locally but is not attached. log messages (currently about 1-2 per hour). The message is from T137551; the usernames are all properly attached though (which means the user impact is small, but it probably breaks the initial central autologin and users have to log in manually on another wiki one more time).
I re-ran the query from T141160#2489124 and it only found four locally unattached users since 2016 Janaury (I fixed those). I spotchecked a few names from the logs and they are all attached. This seems to be some sort of race condition. Maybe SpecialCentralAutoLogin checks the user right after CentralAuthCreateLocalAccountJob has created and attached the user, just when the local connection has been committed but the CA connection still has the transaction open? It seems implausible that it would happen multiple times per hour.