user_login blank for all users

Hello,

I’ve had this odd issue for one of my sites, where all of a sudden (actually not quite sure when it happened), 4 users could no longer login, and when I checked phpmyadmin and wp_users, I noticed that all had a blank user_login. And because of that, a reset login details link couldn’t work.

It all seems fixed for now, but I would very much like to know what this was about, could you help me or point me in the right direction?

Something oddly similar happened to me 6 months ago, and at the time, these sites were hacked… :/