I have had two cases in the last few months where it happened that a new incoming email was routed into one of the existing cases, even though the email should have created a new case.
Our setup is as follows:
Email arrives in shared mailbox -> Subject is read if a CRM token exists -> If yes, the email is routed into the existing case with the corresponding CRM token -> If no, a flow is triggered which creates a new case from the email and pre-fills fields -> Then the case is added to a queue via the routing rule sets.
After a few days, the same customer sent a new email to the same shared mailbox with the subject /Purchase Order / Customer Name / April 11/. Actually, it should have generated a new Case. However, the email was routed to the existing case and appeared on the timeline.