I encounter the same issue, but I did not try to change the keyboard ! At some point, my phone (S7) just crashed, and it now ask me to enter my password to unlock my phone without displaying the keyboard.
On the lock screen, when clicking on the lock button in the middle and switching, the new screen with empty row and "Your device was restarted and need to be unlocked using your password [...]" display, but normally it moves up to display the keyboard. It is just staying at the bottom of the screen, and casually crashing every 10 minutes.
Did someone encounter this problem ?
Have a good day anyway.