External monitor switching to laptop screen when session is locked

When I lock session, the external monitor stops displaying and it switches to laptop screen. Currently I have enabled display setup script so when I start OS it disables my laptop screen and switches to external monitor:

/etc/lightdm/lightdm.conf


display-setup-script=/home/user/display-setup-script.sh

-

/home/user/display-setup-script.sh

#!/bin/sh

#xrandr's name for the external monitor connection
MONITOR=HDMI-0

#bail out if monitor not found
xrandr --query | grep -q "^$MONITOR connected" || ./sbin/prime-offload & exit 0

#the original command that does the switching
xrandr --output "$MONITOR" --primary

OS: Xubuntu 18.04

How can I make that the external monitor wouldn't turn off when I lock session?