Hi,
Please check in the delivery log bounce qualification section, what is the error type mentioned against this error.
Inside the delivery logs of delivery it will indeed appear as Failed. Substatus as Unreachable comes from delivery log qualification rules which is set by deliverability team.
Please check the rules and if it is not per your expectation, please reach out to support so that they can check with deliverability and get it rectified in the central repository.
Regards,
Vipul