Dear Maintainer,

This issue has started happening some time ago, but I don't know when exactly

kscreenlocker sometimes activates too slowly, and systemd seems to be very
eager to suspend.
This results in the previous screen contents briefly showing up before the lock
screen is displayed.

This looks like a broader issue, and apparently it's still unfixed:
https://bbs.archlinux.org/viewtopic.php?id=166221

As a workaround, one can always lock the screen manually before triggering
suspend (through lid close or similar), but this is tedious.