Comment 16 for bug 1806961

Yep, I was able to reproduce here, and it seems like light-locker is not working well with the current version of Gnome (or LightDM) on bionic. My suggestion is that you create a bug on light-locker's github page and try to get some feedback from them. My other suggestion is to install another screen locker if you still want to use LightDM (apt search screen lock ... should give you a pair of tools available). If LightDM is not really a necessity, then perhaps you stick with GDM+Gnome (GNOME DE). There's not much else that we can do from our side, this needs to be taken care by upstream developers in any of the projects (LightDM or light-locker).
Thanks for trying it and helping investigate. If you run into any other problems feel free to report to us in a new bug