SUMMARY
On several freshly installed systems with KDE, the screen saver won't lock the screen while a different virtual terminal is active.

STEPS TO REPRODUCE
1. Configure the screen saver to 1 minute and require the password after additional 5 seconds.

2. While the screen saver in not active, activate a different virtual terminal (e.g. by pressing CTL-ALT-F2)

3. Wait for more than 70 seconds.

4. Switch back to the desktop

OBSERVED RESULT
when switching back, all the open applications (possibly
showing confidential data!) are visible for about one second. Only then the screen starts to dim. And due to the setting of 5 seconds for password requirement,
one can stop the screen saver just by typing some key or moving the mouse.

EXPECTED RESULT
Screen saver is active and only the password screen is visible.

SOFTWARE/OS VERSIONS
I can see the Problem with opensuse-Leap-15.1 as well as with ubuntu-18.04 (when kubuntu-desktop is installed).

ADDITIONAL INFORMATION
Result of mailing list discussion so far:
- Problem can be reproduced by other people
- Problem does appear with sddm, lightdm and kdm
- Problem does not seem to appear with gdm and xfce

Mailing list thread: https://lists.opensuse.org/opensuse/2020-03/msg00267.html