A certificate was issued to someone who isn't the domain owner. Just because the CA can't be blamed because the requester was able to spoof domain validation in a way that the CA can't be expected to detect doesn't mean that a good CA isn't interested in what happened and whether it can somehow be prevented in the future.
One obvious possibility could be e.g. sending a notification to the previous ACME account: "hey, a new ACME account request a certificate for your domain".
