Can't repro on a 2012 retina MBP running 10.13.1, attempting the original repro and others suggested here. Until the wife walks away from hers, it's the only machine I have available. I'm curious as to the difference, given the high number of repros.
Apparently you have to have the password field focused before you submit. Anything in the password field (including nothing) will be saved as the root password.
