In the Kontact Configuration there is an option to select which plugin to start (in view app to start).
Whatever you select, Kontact always start with the plugin which was activated when it was closed.
For example, you select 'Summary' plugin to start always. Apply and Ok. But if you are watching Mail app and close Kontact, the next time Kontact will start it will start with Mail, and if you go to Kontact Configuration, you will see that mail is selected as the plugin to start.
I think it is a bug but I want confirm it before.
Some images: