xtrlock does nothing on second monitor

When I run `xtrlock` on ubuntu 18.10, in gnome-shell with wayland, xtrlock shows its lock on the monitor attached to the laptop but not on the main monitor of the laptop.

xtrlock works as expected on the second monitor, not on the main one:
meaning, one can't type or click anything in the session on the screen on the second monitor, but on the main screen, the mouse cursor is as usual (no lock) and one can type anything in any window.

so xtrlock is inoperative on the main screen.

ProblemType: Bug
DistroRelease: Ubuntu 18.10
Package: xtrlock 2.8
ProcVersionSignature: Ubuntu 4.18.0-12.13-generic 4.18.17
Uname: Linux 4.18.0-12-generic x86_64
NonfreeKernelModules: wl
ApportVersion: 2.20.10-0ubuntu13.1
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Wed Dec 5 13:27:03 2018
InstallationDate: Installed on 2018-05-07 (211 days ago)
InstallationMedia: Ubuntu 18.04 LTS "Bionic Beaver" - Release amd64 (20180426)
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=fr_FR.UTF-8
 SHELL=/bin/bash
SourcePackage: xtrlock
UpgradeStatus: Upgraded to cosmic on 2018-10-19 (47 days ago)