password automatically & incorrectly filling after upgrade

After upgrading to High Sierra, I wasn't able to enter my password. Either the field would not allow me type into it at all (would not recognize typed signals), or it completely auto-filled with a series of asterisks that weren't the same length as my password (which I'd never stored), and reset and autofilled when I hit enter. After several reboots, I eventually I found a recommendation to disable Filevault and got in, but I'm not sure that I want it to be permanently disabled. What gives?