Ubuntu – How to make the login screen appear instead of logging me in automatically

Looks like pam_kwallet.so is missing. PAM is the authentication module used by lightdm and it wants to load kwallet (I guess that's a KDE keyring) but the library is gone.

If you have access (somehow), try to install the package containing the pam_kwalet.so, or try to configure PAM so that it would not look for this file.

Automatically: try running pam-auth-update and disable KDE wallet (kwallet).

Manually: PAM configuration is in /etc/pam.conf and /etc/pam.d/* -- so grep those for kwallet, (re)move configuration referring to kwallet and retry.

Once you are finished with upgrade then you may want to return the pam-kwallet config, just make sure you have pam_kwallet.so installed.

Looks like Ubuntu bug: https://bugs.launchpad.net/ubuntu/+source/lightdm/+bug/1309535

Well, not sure you'll be happy to learn this, but there is what appears to be an active bug that suggests that it's the length of the user name that determines whether to display full name or username, details here: https://bugs.launchpad.net/ubuntu/+source/gnome-shell/+bug/1841810

I'd recommend you test this on your installation to confirm. I can tell you that the full name "John Doe" on my installation (running Gnome on Ubuntu 20.04.1) does show just fine in gdm3.