https://www.jwz.org/xscreensaver/faq.html#gnome-screensaver

22. running GNOME or KDE, and everything's broken! What's wrong?
Probably you are not running XScreenSaver at all, but but are running "gnome-screensaver" or
"kscreensaver" instead. You should stop.
XScreenSaver is secure, stable, and mature; whereas gnome-screensaver is bug-ridden, unreliable,
and an ongoing security disaster (for all the reasons outlined in the On Toolkits page).
Gnome-screensaver was created in 2005 people who thought it was a better idea to rewrite all of
XScreenSaver from scratch instead of sending me patches for the changes they desired in
XScreenSaver itself.
In 2011 or so, "gnome-screensaver" was forked and renamed as both "mate-screensaver" and
"cinnamon-screensaver", re-arranging the deck chairs while fixing none of their fundamental design
problems.
Then in 2013, Ubuntu Unity re-wrote the screen saver engine from scratch again, introducing
numerous security holes that would be hilarious if they weren't so sad.
The only way to securely lock your screen under is to turn off gnome-screensaver, mate-
screensaver, cCinnamon-screensaver, etc. and use XScreenSaver instead. How you about this is
explained in the "Installing on GNOME" section of the XScreenSaver manual.
Remember:
Once is happenstance. Twice is coincidence. Three times is enemy action. Four times is Official
GNOME Policy.