When triggering the passivepopup with kdialog --passivepopup 'This is a notification' 5 it does not hide again after 5 seconds. Kdialog uses the plasma notifications which is nice - is it a problem with plasma? Reproducible: Always
It works here (hides after 5 seconds). Can you check if it works with a freshly created user account? If not, please try to find the setting causing the issue.
I formulated myself badly. Indeed plasma visually hides it automatically - however it does not remove it which I would expect from a timeout. It stays in the queue of notifications. If much used, it will slowly build up a long list of expired notifications together with the valid notifications.
btw. testes on guest account.
The idea is that if you are not sitting in front of the machine, you will be able to see a "backlog" of notifications. I am not sure if there is API to discard a notification completely, once it is posted. Disabling Plasma notifications hides the popup without a way to get it back. *** This bug has been marked as a duplicate of bug 315572 ***