Actually the list of Plugins will not be refreshed if you uninstall a Plugin. The button "Reload list of Plugins" seems to do nothing.
Steps to reproduce:
Uninstall a plugin -> the button text change from "Installed" to "Uninstalled" but is deactivated/grey
Press the button "Reload list of Plugins" -> no visual change
You have to close Picard and restart it to see the real actual Plugin list.
The same is true if you try to install a local plugin. Whatever you try, you can't see your local plugin in the list after pressing the button "Install plugin", browse to the local folder, choose the plugin and press OK. You can see it eventually if you restart Picard.