Seems this is not a new bug. I can reproduce it on Leap 42.1 and SLE12SP2, too. Flipping the autologin switch truncates /etc/sysconfig/displaymanager down from original 91 lines, to 63 lines, then to 31 lines, at this point the line containing DISPLAYMANAGER_AUTOLOGIN itself is dropped, and resulting in further switching not possible. Possibly a bug in accountsservice/src/gdm-sysconfig.c gdm_sysconfig_save_value(). But I haven't dug into it yet.