Users might run into a scenario where a user that was CC'd on the thread, and replied to the conversation is not being added to the CC field when an agent uses the Reply option.
As seen in the screenshot below, the CC user (firstname.lastname@example.org) is not being added to the CC field and only the requester address and the other CC user are there.
The behavior described above is by design. Using the reply button means that the message will be sent to the email address of the one that responded to the conversation. Thus, adding the user e-mail to the CC fields as well would be redundant.
In the case of the screenshot above, the email would be sent to email@example.com. Then, the ones that are automatically added in the CC field are the conversation requester (firstname.lastname@example.org) and any other emails(email@example.com) that were copied in the response from the user.