KDE does not display screen lock after suspension

[Infthi]

Reproducible:
always

Steps to reproduce:
1. Use a laptop.
2. In system settings, enable "Desktop behavior > Screen Locking > Lock screen on resume".
3. In system settings, set "Power management > Energy saving > When laptop lid closed" to "Suspend".
4. Close laptop lid. Wait your laptop to suspend.
5. Open laptop lid.
6. Log in into system.
7. Enable any context menu (e.g., right-click on the browser titlebar)
8. Close laptop lid. Wait your laptop to suspend.
9. Open laptop lid.

Observed behavior:
A screen lock appears after step 5, but not after step 9.

Expected behavior:
A screen lock should appear both after steps 5 and 9.

System information:
Description: Ubuntu 16.04 LTS
Release: 16.04
(Actually this is a kubuntu installation)

KDE PLasma version: 5.5.5
Qt Version: 5.5.1
Kernel: 4.4.0-28-generic #47-Ubuntu SMP Fri Jun 24 10:09:13 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux

Additional information:
This entries appear in ~/.xsession-errors during suspend/wake:

powerdevil: Suspend session triggered with QMap(("Explicit", QVariant(bool, true))("Type", QVariant(uint, 1)))
powerdevil: Suspend session triggered with QMap(("Explicit", QVariant(bool, true))("SkipFade", QVariant(bool, true))("Type", QVariant(uint, 1)))
powerdevil: Starting Login1 suspend job
lock called
bluedevil: About to suspend
Could not establish screen lock
"No such interface 'org.freedesktop.DBus.Properties' on object at path /"

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[LandRacer (nick-sutton)]

I can confirm this bug.... Just noticed it. Seem's like a PRETTY SEVERE security risk.... Not sure why this hasnt been fixed. The issue was reported on 7/7/16 and its now 4/19/17. I'm wondering who allowed this into the wild to begin with??? Your fired... PERIOD! An LTS release is NOT SUPPOSE TO BE AMATEUR HOUR!!! WTF? This bug needs dealt with NOW or I'll be moving away from KDE to something more secure. Get your act together girls!

My system info

ACTUAL Kubuntu install of 16.04 LTS
kernel 4.4.0-72 x86_64
plasma ver 5.28.0
qt ver 5.6.1

[LandRacer (nick-sutton)]

How Do I reproduce???

Well any window any time... right click, close laptop lid. Wait for suspend, re-open. NO LOCK SCREEN, EVER! NO MATTER HOW MANY TIME YOU DO IT... EVERYTIME!!!

FIX IT NOW!

[Salamander (taav1-23)]

Okay, yeah, this is not okay. Fix please.

The screen **does** get locked if there's a screen lock timeout set but without it it comes out of suspension without a lockscreen which is **not** how other DEs/OSs work and should the very least be warned against.

[Rik Mills (rikmills)]

Not fixable for X. Does not occur on Wayland.

See: https://bugs.kde.org/show_bug.cgi?id=383327

and https://bugs.kde.org/show_bug.cgi?id=70063