I have just installed Lubuntu 20.04. I see this

$ sudo update-alternatives --config x-session-manager
There are 2 choices for the alternative x-session-manager (providing /usr/bin/x-session-manager).

  Selection    Path                      Priority   Status
------------------------------------------------------------
* 0            /usr/bin/startlxqt         50        auto mode
  1            /usr/bin/openbox-session   40        manual mode
  2            /usr/bin/startlxqt         50        manual mode

Press <enter> to keep the current choice[*], or type selection number: 

$ wmctrl -m
Name: Openbox
Class:
PID: N/A
Window manager's "showing the desktop" mode: OFF

Is there any contradiction in the fact that update-alternatives shows startlxqt which is likely what is used, and wmctrl returns Openbox?
Is this indicative of any configuration issue?
How can this be explained?

nobody: right, openbox is the Window Manager, and LXQt is the session manager. What possibly helped in my confusion is that I didn't know openbox had a session manager.

$ sudo update-alternatives --display x-window-manager
x-window-manager - auto mode
  link best version is /usr/bin/openbox
  link currently points to /usr/bin/openbox
  link x-window-manager is /usr/bin/x-window-manager
  slave x-window-manager.1.gz is /usr/share/man/man1/x-window-manager.1.gz
/usr/bin/openbox - priority 90
  slave x-window-manager.1.gz: /usr/share/man/man1/openbox.1.gz

Solution 1:

LXQt is a desktop; it's WM agnostic by design (unlike some DEs that were designed for specific ones), but expects one for full functionality.

Lubuntu uses openbox for that purpose, but not all do (eg. Debian use xfwm4 to perform that role with LXQt installs).

You can switch it to another if you desire (see the manual - https://manual.lubuntu.me/lts/3/3.2/3.2.13/session_settings.html) though some keyboard shortcuts Lubuntu adds require openbox so those will stop working if replaced.

I see no contradiction(s). Openbox can be used on it's own if you wish, or LXQt be used with another WM and not Openbox.

wmctrl of course is looking at the WM component; which is openbox for Lubuntu, as it's not a desktop supported feature.

Your issue is comparing/expecting a desktop that is linked to a certain WM (GNOME with mutter, Cinnamon with muffin, etc), and LXQt, which is modular allowing end-users to configure it to be used by other components as users decide (Lubuntu just uses openbox).

FYI: Some LXQt devs don't like openbox either; so if they decided to stop being agnostic & link LXQt to a WM, I doubt it would be openbox.