light-locker causes tty to change

After upgrading to 18.04, on xfce4 with Display Power Management (xfce4-power-manager) enabled, or any of the three sliders set to non-zero, then when the time limit is hit after inactivity two monitors power down, but then the tty switches to tty8, and the third monitor just shows a black with a cursor flashing. Switching back to tty7 shows text saying that the session is locked, but no amount of keyboard or mouse activity brings up a prompt to unlock it.

Two Xorg logs are created when this happens (see attached). Ctrl-Alt-F# will switch to other ttys at this point, but only briefly before switching back to the one that is black with a single flashing cursor. After a few switchings between ttys, it stops automatically going back to tty8.

This problem also occurs when xscreensaver is used to control the display power management as well. However on switching back to tty7 in this case, the unlock dialog is presented as expected and things continue to work. So it would appear it is something more fundamental than xfce4-power-manager that is causing the active tty to change when the screens go sleep.

The screen power off that occurs while the lightdm's graphical login is presented (i.e. before logging in) works fine.

This problem appears to be due to light-locker. If I stop it from running the problem goes away.

(I would expect things to happen as in 17.10, namely that the displays blank/switch off as selected, and then when further input is given they unblank/switch on.)

Please let me know what further information I can provide to narrow down the problem.

lsb_release -rd:
Description: Ubuntu 18.04 LTS
Release: 18.04

apt-cache policy xfce4-power-manager:
xfce4-power-manager:
  Installed: 1.6.1-0ubuntu1
  Candidate: 1.6.1-0ubuntu1
  Version table:
 *** 1.6.1-0ubuntu1 500
        500 http://gb.archive.ubuntu.com/ubuntu bionic/universe amd64 Packages
        100 /var/lib/dpkg/status

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: xfce4-power-manager 1.6.1-0ubuntu1
ProcVersionSignature: Ubuntu 4.15.0-29.31-generic 4.15.18
Uname: Linux 4.15.0-29-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.20.9-0ubuntu7.2
Architecture: amd64
CurrentDesktop: XFCE
Date: Wed Jul 25 21:10:33 2018
InstallationDate: Installed on 2017-05-13 (438 days ago)
InstallationMedia: Xubuntu 17.04 "Zesty Zapus" - Release amd64 (20170412)
SourcePackage: xfce4-power-manager
UpgradeStatus: Upgraded to bionic on 2018-07-16 (9 days ago)