If a user changes a default desktop background, a login screen will from now on revert to the default background after appr. 1-2 sec. interfering with typing a password. A simple solution is to uncomment "#background=" line in [greeter] section of /etc/lightdm/lightdm-gtk-greeter.conf and providing a path to the selected background file, but I don't think a user should be forced to do this and instead LightDM configuration should be altered automatically.
The bug you're describing has nothing to do with Xfce or xfce4-session in particular. It seems to be an issue with LightDM, potentially light-locker and VT switching in general.