Emails containing square brackets [] in the subject line confuse GMail. When the conversation view (default) is used GMail ignores anything inside square brackets. Like the standard ticket id that is formatted like this: [#{{ticket.id}}].
This means that all email with the same subject line is added to the same conversation, because is has not been differentiated by the ticket id. This is probably not what you would want when referring to separate tickets. This occurs for us as we use website forms to create tickets - they have the same description in the subject line.
One possible solution would be to allowing email subject line templating everywhere so we can choose the format of the subject text as needed by the use case.
With the system as it is today this would involve changing the email reply template and adding an email forwarding template.
