This happens to me after the mac has been used for a while (switching between my wife's account and my own). Its infuriating since I can't log on without her losing her work so it makes the computer useless for me if she isn't available.
To be clear this isn't a problem of clicking as some has suggested, since when the bug occurs it doesn't matter if you use the mouse, or use the keyboard to tab select the user, either way it will keep going back to the other user after you select the one you want. Touch ID won't work either.
Can't believe apple haven't fixed this already... its a very serious issue that makes the system useless for our family.