Hello,
I'm sorry because I did not provide an explanation.
This little loginctl--lock-sessions.service systemd unit is only invoked when computer reached the systemd sleep.target which is called when computer goes to sleep, suspend, hibernate, and hybrid-sleep using systemd.
loginctl--lock-sessions.service is never invoked by the screensaver.
You can rename the loginctl--lock-sessions.service by using the name you want.
loginctl--lock-sessions.service rely only on systemd mechanism and just launch logger and loginctl lock-sessions. Unit is invoked by the sleep.target which is called by systemd-hibernate.service, systemd-suspend.service, systemd-hybrid-sleep.service ... without additional package ... only the base system.
As example: systemd-hibernate.service
This file is part of systemd.
systemd is free software; you can redistribute it and/or modify it
under the terms of the GNU Lesser General Public License as published by
the Free Software Foundation; either version 2.1 of the License, or
(at your option) any later version.
[Unit]
Description=Hibernate
Documentation=man:systemd-suspend.service(8)
DefaultDependencies=no
Requires=sleep.target
After=sleep.target
[Service]
Type=oneshot
ExecStart=/lib/systemd/systemd-sleep hibernate
This unit do the following actions:
First one :
ExecStartPre=/usr/bin/logger --tag "loginctl" --priority auth.info "Session: Locking all user sessions"
This will cause logger to write to the journal log an event:
juil. 05 04:46:46 ...... loginctl[20960]: Session: Locking all user sessions
Second one:
ExecStart=/bin/loginctl lock-sessions
loginctl --help
lock-sessions Screen lock all current sessions
So after restoring your computer from suspend or hibernate you can search in journal log if loginctl--lock-sessions.service has been launched by the sleep.target and if the sleep.target has been invoked ... using journalctl
Using the logger tag:
$ journalctl -t loginctl
Using the systemd unit name:
$ journalctl -u loginctl--lock-sessions.service
This is only a little workaround to force screen to be locked on sleep.
You can replace loginctl lock-sessions with whatever you want coming from additional screen locking tools.
Regarding the mate screensaver (inactivity screensaver), i don't know what to say ... because I have troubles with it ... sometimes screen is well locked and sometimes not 
.... so now WIN+L is a reflex !
Kr,
picked a good deal of fixes from MATE Desktop development snapshots. In total, 67 additional patches have been applied to the MATE Desktop packages in Ubuntu MATE 19.10 to finesse this release prior to launch day 
Included in those patches are fixes for locking the screen on resume from suspend [...]