If new updates are found (manual "Check for Updates") after updates have been applied, the "Install Updates" button is not re-enabled but remains greyed out.
To re-enable the "Install Updates" button, updater needs to be closed + opened again.
When I start the updater manually after starting kubuntu and logging in, and run "Check for Updates" and updates are found, the "Install Updates" button is always disabled. Usually at the same time, I get a (IMO redundant) notification that updates are available. If I then close the updater instance that I manually started and click on the notification to start another instance of the updater, the "Install Upgrades" button is enabled in that instance.
I've fixed this for 2.0.1.