Back when I used Window$, I often copied text from one app, closed it, then pasted the text into another app. But in KDE, if you close the source app before you paste the text, it disappears.
This happens because X doesn't impose policy on apps. It is the responsilbility of the applications to handle cut/paste. In other words, when you paste, the app which has the copied text sends it to the receiving app. If you close the app, then the selected text disappears.
So if you want selections to remain after the app is closed, then you need a separate clipboard app, such as klipper.