The workaround appears to be to delete the entire order, and the entire event with associated tickets (including permanent deletion from trash). I haven’t reviewed the database, so can’t say whether this course of action has cleaned that up properly.
This is not acceptable for a live site, because:
(1) It’s not unreasonable to require a record of orders, including those that are paid and refunded. The above course of action destroys the order, when a cancelled order just needs to trigger a re-stocking of the inventory (add the ticket(s) back) and a removal from the attendee list.
(2) Deleting a whole event plus tickets is not feasible for an actual event where tickets have already been purchased.
Please confirm whether you agree that this is an issue, and whether you will be addressing it. Many thanks.