Configure light-locker for xubuntu 18.04?

I am running xubuntu 18.04.

I recently installed the nvidia proprietary drivers.

I have three monitors.

When I lock my screen using light-locker my left-most monitor turns off and the other two monitors mirror each other at a 800x600 resolution.

I had a similar-but-not-identical problem with my login screen, which I solved: How to adjust display settings for LightDM greeter on 18.04?

How do I apply a similar solution to my lock screen? I'm assuming it's a different configuration file but I can't find where it might be.

Here is the output of light-locker --debug, if it is helpful.

[gs_debug_init] gs-debug.c:106 (21:58:56):   Debugging enabled
[main] light-locker.c:142 (21:58:56):    initializing light-locker 1.8.0
[main] light-locker.c:164 (21:58:56):    Platform:
gtk:        3
systemd:    yes
ConsoleKit: yes
UPower:     yes
[main] light-locker.c:196 (21:58:56):    Features:
lock-after-screensaver: yes
late-locking:           yes
lock-on-suspend:        yes
lock-on-lid:            yes
settings backend:       GSETTINGS
[main] light-locker.c:198 (21:58:56):    lock after screensaver 0
[main] light-locker.c:199 (21:58:56):    late locking 0
[main] light-locker.c:200 (21:58:56):    lock on suspend 0
[main] light-locker.c:201 (21:58:56):    lock on lid 0
[main] light-locker.c:202 (21:58:56):    idle hint 0
[init_session_id] gs-listener-dbus.c:2193 (21:58:56):    Got session-id: /org/freedesktop/login1/session/c2
[init_session_id] gs-listener-dbus.c:2198 (21:58:56):    Got sd-session-id: c2
[init_seat_path] gs-listener-dbus.c:2279 (21:58:56):     Got seat: /org/freedesktop/DisplayManager/Seat0