Case email from address defaults to User's email after modifying Email-to-Case routing address

This is expected behavior in Salesforce Email-to-Case. When an Email-to-Case routing address is modified, any existing cases that were originally created using the old routing address retain a reference to that (now invalid) email address. Because the old address no longer exists as a valid routing address, Salesforce cannot use it for outbound emails from those cases. Instead, it falls back to the logged-in user's email address and places the former routing address in the CC field.
No configuration change is needed to resolve this — it is working as designed. If you want to prevent this behavior for future cases, create a new routing address rather than modifying an existing routing address that is already associated with active cases.