steps to recreate:
create new system admin, give them a username and password and hit save.
looking at the list of sys admins, edit your new user.
select the user and then select login access.
you're taken back to the change password screen and are being required to enter the old password.
enter the current password and then try to add an IP. dialog box pops up to enter IP, click save, dialog box disappears but there is no enter for the IP you just tried to add.
running 12.3.5318