User missing in unity-control-center "Users panel" (Solved strangely)

One day (2020-12-9), I found there are no any user listed in "User panel" inside unity-control-center (also gnome-control-center).  It's sort of frustrated! Google any related terms, found this link:

https://bugs.launchpad.net/ubuntu/+source/gnome-control-center/+bug/1876460https://bugs.launchpad.net/ubuntu/+source/gnome-control-center/+bug/1876460

I followed some messages in there to create another system user.  Like this,  ( "joe" was already a user. )

sudo userdel -rf joe
sudo adduser joe
sudo adduser joe sudo 

chiao@kpci7:/etc$ id joe
uid=1000(joe) gid=1003(joe) 群組=1003(joe),27(sudo)

The strange thing happened. I open the "User panel" in unity-control-center again. All the users were back! Like this: