, the value of this field is an internal field.
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Internal system identification of the vehicle. Should be unique per vehicle, and is used for tracking the vehicle as it proceeds through the system. This id should not be made visible to the end-user; for that purpose use the label field [emphasis added].The MTA has not changed the industry specification, and their data has remained compliant with the specification throughout this time. The endpoint clearly defines the value you were using as an internal identifier, distinct from the user-visible `label` field. Insofar as there is correspondence between keys across fields, the structure is valid, regardless of the value's content (be it a random hex string, potato, 1234, etc.). These fields could change at random and would not warrant notice.
Per your own message, your decision to make an inference from a field's content in direct contravention of the specification was done at your own business risk.
Accordingly, please convey this as the root cause to your stakeholders.
[Disclosure: I am not an MTA representative and am speaking as a fellow member of this group frustrated by this interaction. There are plenty of opportunities for fair criticism of the MTA, but this is not one such instance. Continuing to press the issue disincentivizes the MTA to actively participate and address actual issues or discrepancies.]