This is working as designed. The send event will trigger for posted tasks, notes and appointments in GroupWise 6.5.2. (From memory the event didn’t work in earlier versions of the GroupWise client – this was considered a bug).
As there is no ‘On Post’ event, triggering the On send event is logical when some post-processing needs to be performed when an item is initially created. The event does not trigger when subsquent updates are applied to the same item and it is re-posted.