I use a password manager and auto-login program (1PassWord) and don't remember seeing that dialog-box before.
I thought there might be a limit to active or connected users and you'd get the boot if you remained inactive to preserve user-slots. I never noticed it before, to be honest and thought it was new.
[EDIT] Problem solved. It's not a bug. I logged in like a normal human being, saw the dialog-box and unticked the "Stay logged in" check-box