After a security update (red) the taskbar icon continually displays "updates available" (blue) when they are none.

There have been two sets of updates in the last week that are security ones, so the taskbar icon has a red dot.
After clicking on this and installing the updates Discover reports there are none. However, the taskbar icon continues to say that there are updates (now with a blue dot).

But both the Discover GU and apt-get update/apt-get upgrade say there are none.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: plasma-discover 5.12.4-0ubuntu1
ProcVersionSignature: Ubuntu 4.15.0-20.21-generic 4.15.17
Uname: Linux 4.15.0-20-generic x86_64
ApportVersion: 2.20.9-0ubuntu7
Architecture: amd64
CurrentDesktop: KDE
Date: Wed May 16 17:42:54 2018
InstallationDate: Installed on 2018-05-09 (6 days ago)
InstallationMedia: Kubuntu 18.04 LTS "Bionic Beaver" - Release amd64 (20180426)
SourcePackage: plasma-discover
UpgradeStatus: No upgrade log present (probably fresh install)