Running Kubuntu 18.04 the solution for me was adding

```
export XDG_DATA_DIRS="$XDG_DATA_DIRS:/var/lib/snapd/desktop/"
```

to a custom file like `snap-apps.sh` in `~/.config/plasma-workspace/env`. But that applies only to Plasma 5 and might be fixed in Ubuntu 18.10 already.

[https://askubuntu.com/a/1086249](https://askubuntu.com/a/1086249)