suspend does not lock in 18.04

To reproduce from the command-line (this is ubuntu 18.04):

% gsettings set org.gnome.desktop.screensaver ubuntu-lock-on-suspend true
% gsettings get org.gnome.desktop.screensaver ubuntu-lock-on-suspend
true

% systemctl suspend

Now the monitors go black and report "no signal" (good), and it will stay that way for hours (good) ... but if I tap "shift" key a few times, Ubuntu 18.04 comes back without asking my password. (I expected rather that the system would be locked, i.e. I should see a dialog box that would require a password before anything else!)

Same thing if I suspend by using the upper-right-of-screen "power" icon (click-and-hold, then click on the suspend icon).
Same thing if I let 20 minutes go by. (A msg comes up ~~ "system will suspend soon...", and it does suspend, but doesn't lock).

I have left the ubuntu-lock-on-suspend setting at "true" through several re-boots, but it still doesn't take effect.

I get the same behavior whether ubuntu-lock-on-suspend is set to "true" or "false".

Other info:
% wmctrl -m
Name: GNOME Shell
Class: N/A
PID: N/A
Window manager's "showing the desktop" mode: N/A

% lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 18.04.1 LTS
Release: 18.04
Codename: bionic

% uname -a
Linux ucbvax 4.15.0-36-generic #39-Ubuntu SMP Mon Sep 24 16:19:09 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

I just upgraded from 16.04 a couple of weeks ago. I think (90%) it was locking in 16.04, but now not locking in 18.04.

If I click on the upper-right-of-screen padlock icon, then the system does lock (password required).

While it's locked in that way, I can click on the upper-right-of-screen "suspend" (from drop-down), and then my system is both suspended and locked. But the question is ...

How can I make my system lock (automatically) every time a "suspend" happens? (Or even "every time kbd and mouse are idle for 15 minutes"?)

Please advise.
thanks