I don't remember if it cleared them or not. One of things having no-testing help for Dunamis made me miss :P
I assume you added the DB_EventSet(_Event) to the DialogStarted statement as well? You might want to double check if that isn't causing too much delay on the procedures. For instance in my new mod I have a few checks that take place during the greeting, and if the timing is off even a little bit the events don't get set in time.
One workaround for this is I've added a buffer dialog line so there would be one line of dialog like:
NPC - Oh Hello There! - no check
Press continue....
NPC - Wow you look strong, maybe you can help me with....-Strength 8 check.
You could try putting the clear event on DialogStarted as well, and run it in a chain of procedures right before the dialog. Though that might cause even more delay/lag.
Dialog ended is probably the best though, but just kicking around a few extra ideas.
