xscreensaver appears only on one of two monitors

I have a Debian Bullseye system, ASUS PN53 (built in Radeon graphics) with two monitors, one connected via HDMI and one using a USB-C adapter.

When xscreensaver locks the screen, only the HDMI monitor displays a screen saver. The USB-C monitor continues to show whatever windows were on it, although the screen lock works to the extent that you can't interact with them until you unlock.

Mate-screensaver worked correctly, but xscreensaver is superior, and also I'm now using Xfce-4.

Suggestions?