CC'd users replying to a ticket notification are creating new tickets instead of adding to the existing thread.

The issue was that the tickets in question had a 'Closed' status that was configured to not allow reopening on reply. Adjusting the ticket status settings so that only 'Resolved' tickets cannot be reopened on reply (while 'Closed' tickets can be) fixed the problem. To debug further: search the ost_thread_entry_email table for the Message-IDs in the References/In-Reply-To headers of the offending email -- if a Message-ID is not found, osTicket cannot match the reply to an existing thread.