I tried xscreensaver 5.45 and it doesn’t fix the problem, so I expect we will have to wait for 6.02.
But I’m not holding my breath seeing as the developer doesn’t get along too well with the Debian guys.
Disabling xscreensaver and using only the power manager (lxqt) doesn’t work either.
If you don’t mind just having a blank screen (without the screen saver), you can install xfce4-power-manager and use that instead (xfce4-power-manager-settings) .
But you will have to replace the autostart;
Preferences > LXQt settings > Session Settings >
Autostart > LXQt Autostart > XScreenSaver >
Edit > Command
Change: xscreensaver -no-splash
To: xfce4-power-manager
OK
Logout/Login
And also disable the lxqt power settings (lxqt-config-powermanagement).
The desktop shortcut for xfce4-power-manager config is in
usr/share/applications/xfce4-power-manager-settings.desktop
This setup tested ok with smplayer (media player) and opera (web browser based on chromium),
but it still doesn’t work with firefox (don’t know why).
(I also tried xfce4-screensaver, but that doesn’t let itself be inhibit either, it only seems to work with xfce4-power-manager alone.)
(Lubuntu 20.04.02-3)