Please be so kind and open a bug report for this behavior on http://bugs.otrs.com 

Doing so right now.

For the meantime I think you should be fine with selecting ticket states as well as a kind of restriction.
If there is a new Ticket the state will be “new", otherwise it should be something different.

I didn't even think about adding those. And that is a very acceptable compromise in the meantime. I've set the TicketCreate notification to also include State: New, and the TicketCustomerUpdate to also include every state EXCEPT new.

Thanks much, Nils.