For anyone else encountering this and using Google Groups to route your centralised helpdesk emails to a service account for pickup by the Google/IMAP channels, this will add the Precedence: list header to the email, meaning that Zammad marks it as send_auto_response = false and is_auto_response = true
To workaround this either just add the email address directly as an Alias to the service account, or add an Internal - Sending route in workspace from the Helpdesk Group and add the Zammad Headers:
You’ll also need to add your Google channel as trusted:
https://admin-docs.zammad.org/en/latest/channels/email/email-headers.html