Forwarded incidents updating existing incidents instead of creating new incidents

Content

We upgraded to 18A recently, and since then we've been having a strange problem. 

If we've a ticket open with one department, lets call them: DeptA and they have an incident with the mailbox: [email protected]

Things get completed by that department, and the incident is closed. But the customer continues to send the email around, forward it, etc... eventually they need different help from a different department... lets call them DeptB with a mailbox of [email protected]

Prior to our upgrade, when they forwarded the old incident into this different mailbox, it would create a new incident.