This article explains why the bill of lading is not automatically populated when tracking by carrier code
By default, the payload response when tracking by carrier code will not automatically provide the bill of lading if it was not manually provided when creating that particular reference.
When tracking by a singular container number, the data does not return a value for the master bill of lading by default. In terms of establishing a hierarchy, knowing the BL number ranks higher than a singular container ID. A carrier does not typically share the BL value back to us when we receive the data for a container based only by its ID.
If a user wishes to track a container and can also provide the BL, it is highly recommended to do so. This is accomplished by utilizing the optional 'bill_of_lading' parameter of the API's Create Reference via Carrier Code endpoint.