Software Center Not Showing Installed Apps
I had the same issue.
There was a conflict with the snap store.
Running killall snap-store in a terminal then relaunching the app fixed it.
Restart your PC!
Same problem after fresh install of Ubuntu 20.04, there are no categories in the software center.
What I did: opened the terminal (Ctrl+Alt+t), then
sudo apt install --reinstall gnome-software
I tried to open software center, still the same. Then, restarted my machine and voilà:
I don't know if it is worth mention but I think in the beta version software center was fine. I don't remember having problems.