This is a known bug with Plasma that is reported to be fixed in a newer version of Plasma 5.16.0 which is not available by default on Kubuntu 19.04. A quick search shows that it is not available in 18.04 either, so it is likely affected as well
Kubuntu 19.04 is currently running 5.15.4 (and 18.04 on 5.12) so you may encounter this annoying bug.
The bug is related to flatpaks. If you have flatpaks enabled in KDE's Discover, at least some packages are not update-able via Discover.
You can easily workaround the issue and fix in the terminal by running
This will update the outdated flatpak that Discover was unable to update and should make your persistent notification about a "phantom" update go away.
In this case, the flatpak affected was an Nvidia driver but the bug reports mention others as well.