No. That wasn’t the case on bionic several updates ago (or with ubuntu-unity-amd64.iso) at-least for single user.
I think It has something to do with permissions. For example with latest g-c-c 3.28 creates ~/.cache/gnome-control-center/backgrounds/ with root. These settings are also important
gsettings set com.canonical.unity-greeter draw-user-backgrounds true
gsettings set org.gnome.settings-daemon.plugins.background active true
But even after fixing permission the issue exists. Though /var/lib/AccountsService/users/ properly shows the backgrounds section.
I have started experiencing this problem since updating gnome components to 3.28