I've installed everything from central software/update -> now it says there is nothing to install and gives me an option to disable some filters. Clicking that produces the following dialog:

The first strange part is, I actually need to uncheck the 'Show installed' checkbox in this dialog to show the installed plugins. It might be technically disabling the filter, but it looks weird.

The second strange thing is, after 'disabling the filter' it shows the installed plugins, but the 'Show installed' checkbox is still unchecked.