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