- address standardization is enabled
- an incorrect USPS USERID is configured
- your USPS USERID has not been granted access to the USPS production server
If you enter a new contact or update an existing contact under these conditions, the contact's address fields are cleared, and the CiviCRM UI does not report any kind of problem.
The USPS does notify of authentication issues in the XML they return. CiviCRM should see those, report them in the UI, and not update the contact.