I use KOrganizer to sync events with my Google Calendar. It now happened a few times that I deleted an entry in the agendaview and it was still there afterwards. (It only loses the Google 'g' in the upper left corner.) I have no problems when deleting a local-only event. Since the event is still showing up I can also move/stretch it, but not delete (Del key) it. Also, the context menu won't show up when right clicking on it. When showing the preceeding week and then coming back again the event is gone. Note: Menu -> View -> Refresh does *not* delete the event, I really need to change the currently displayed week and come back. It seems as if this only happens to newly created events. When I create an event, switch the week and come back and then try to delete it, this does not happen.
I found that, if one deletes the created event in Google Calendar other than agendaview, symptom is the same as described above. I am current using KDE Applications 18.12.2. P.S. Thanks for showing that switching week and returning back could make the deleted event disappear from KOrganizer.