member account disappear on random

Hi There,

One of my members, account name is babychu, disappeared from my member list around lunch time on July 23. The member can still post, but it shows up as guest_anonymous on the forum.

I've checked with the founder, all admins and mods, none of them removed the account. In fact, the member tried reapplying and was refused because the account/email already exists.

Could you please check into this?
Thanks.