Hi,
In SCSM 2022, the behavior you're describing is indeed the default. When an email is sent to the integrated mailbox with the existing incident number in the subject line, the email is attached as a related item, and an entry is added to the Action Log as an End-User Comment. However, the incident remains in the Resolved status and does not automatically re-open.
To change this behavior, you would need to customize the workflow. Unfortunately, there isn't an out-of-the-box setting to auto-reopen resolved incidents when they are emailed by the end-user.