A look-and-feel package may specify its preferred default wallpaper in ./contents/defaults as

[Wallpaper]
Image=<wallpaper kplugin>

We use this in Fedora to set our default wallpaper, and it used to work until Plasma 5.27
In Plasma 6 alpha, it doesn't seem to have any effect anymore.

STEPS TO REPRODUCE
1. sed -i -e 's|^Image=.*$|Image=Kite|g' /usr/share/plasma/look-and-feel/org.kde.breeze.desktop/contents/defaults
2. Select the Breeze look-and-feel, applying the Desktop layout too. 
    Or, preferably this is already the default look-and-feel package)
3. Log out
4. Log in

OBSERVED RESULT
The wallpaper is still "Next", both for existing and newly-created users

EXPECTED RESULT
The wallpaper is "Kite"

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: Fedora Rawhide
(available in About System)
KDE Plasma Version: 5.27.80
KDE Frameworks Version: 5.245.0
Qt Version: 6.6.0

ADDITIONAL INFORMATION