Another screensaver daemon is started in conjunction with locking

For example you may have set up only xscreensaver to be triggered on login in session autostart. When xfce4-power-manager locks screen before sleep, it starts xfce4-screensaver, if it is installed.

This is a kind of rebirth of the bug that was reported over 13 years ago: https://bugzilla.xfce.org/show_bug.cgi?id=3770

Use the running screensaver. If xflock4 script is present (via xfce4-session), you can use that.

Tested in: xfce4-session 4.14.2 (Xfce 4.14)

xfce4-session has similar issue xfce4-session#108 (closed)