I believe I have the same problem and I'm on v1.17.2.
I create a ticket and add a CC user.
Both the owner and the cc user get the new ticket email.
If the cc user replies, the agent gets the alert but not the owner.
If the owner replies, the agent gets the alert but not the cc user.
In other words, " Participants: Send new activity notice" isn't working. All participants should be getting notified of new activity whenever either the owner or CC user replies but thats not happening.
Update: I have found this is only an issue when a user replies to an email and it is "fetched", rather than a reply directly to a ticket via osticket. I raised this issue: https://github.com/osTicket/osTicket/issues/6413