My Ubuntu Software doesn't show installed programs or available programs any more
Solution 1:
Since this is a known bug which Canonical must fix, may I offer an alternative until then? https://help.ubuntu.com/stable/ubuntu-help/addremove-install-synaptic.html.en describes the Synaptic Package Manager which is described in the link above as more powerful and can do some software management tasks which Ubuntu Software can't.
Solution 2:
You can fix some problems with:
pkill gnome-software
sudo rm -r /var/cache/app-info
sudo appstreamcli refresh --force --verbose
These lines solved my problems :-)
Found the above commands in this answer to Why is Ubuntu Software Center dropping previously provided applications? by Terrance.