The customer has a Data protection policy with the "Tag the subject" action, set to trigger on Outbound or both Outbound and Inbound emails.
For outbound emails, the subject line isn't tagged and the email is delivered, while Track and Trace will display the Data protection policy triggering and the action take Tag the subject.
This is by design. Indeed, such action on outbound emails could disclose part of your Data Protection policies to external parties, hence providing insight to your internal security policies.
This behavior cannot be changed. It is mentioned in the service overview, in "Step 4".