How to change GDM3 login screen background

Hi,

I'm trying to update my gdm3 login screen background.

I followed the instructions here: https://wiki.debian.org/GDM :

Customizing the GDM appearance
GDM v3.0+ allows for some basic customization, such as changing the logo icon, display background, and GTK theme. To do so:

Edit /etc/gdm3/greeter.dconf-defaults as root

Uncomment and/or modify the desired settings
Save and close the editor
Finally, run as root: dpkg-reconfigure gdm3

The command 'dpkg-reconfigure gdm3' will cause gdm to reload its configuration upon the next logout or reboot.

Note: When changing the background image, the target file must be world-readable.

But this did not work, I still have the default background.

Here's my greeter.dconf-defaults (other changes here have been picked up):

aclark@pleco 0 /etc/gdm3 $ egrep -v '(^#|^$)' greeter.dconf-defaults
[org/gnome/desktop/interface]
[org/gnome/desktop/background]
picture-uri='file:///usr/share/backgrounds/TNAwallpaper4K.jpg'
picture-options='zoom'
[org/gnome/login-screen]
logo='/usr/share/images/vendor-logos/logo-text-version-128.png'
disable-user-list=true
[org/gnome/settings-daemon/plugins/power]

The background exists and is world readable:

aclark@pleco 0 /etc/gdm3 $ ls -l /usr/share/backgrounds/TNAwallpaper4K.jpg
-rw-r--r-- 1 root root 2310969 Apr 3 10:34 /usr/share/backgrounds/TNAwallpaper4K.jpg

Running testing:

aclark@pleco 0 /etc/gdm3 $ cat /etc/debian_version
buster/sid

Checking gdm3 is actually running, not light dm or something:

aclark@pleco 0 /etc/gdm3 $ ps -ef | grep '[g]dm3'
root 942 1 0 11:24 ? 00:00:00 /usr/sbin/gdm3
aclark 2779 2756 0 12:05 tty2 00:00:00 /usr/lib/gdm3/gdm-wayland-session /usr/bin/gnome-session

gdm3 version:

aclark@pleco 0 /etc/gdm3 $ dpkg -l gdm3
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name Version Architecture Description
+++-==============-============-============-=================================
ii gdm3 3.30.2-3 amd64 GNOME Display Manager

What am I missing?

Cheers,

Andrew.