I was able to reproduce your situation after uninstalling plugin that installed manually, and I noticed that the folder com.github.casout.wallpaperEngineKde in ~/.local/share/plasma/wallpapers/ has been deleted.
I checked and nix had installed it on /run/current-system/sw/share/plasma/wallpapers and KDE cannot seem to recognize.
Finally I solved this problem by adding a small script. I would like to know if there is a better way to add the folder in user .local directory.