The letter is not void because its signer left, but it becomes harder to stand behind if no one at the employer can confirm it. TN rests on prearranged professional employment with the employer, and a letter nobody can verify weakens the evidence of that arrangement.
Evidence follows the employer, not the signer
What matters is that the U.S. employer has arranged specific professional work matching a listed profession, and that the documentation reflects that accurately. A signature is one form of confirmation, not the arrangement itself.
Where the signer has left, the practical remedy is a current signature from someone with authority, kept alongside the original for continuity. The failure that surfaces first is a mismatch nobody planned: the reissued letter describes the job in the successor’s vocabulary, and the two versions no longer name the same profession. One record settles that, and it is the original job requisition, because it predates both signatures and states the title, the qualification required and the duties as the company itself defined them.
Read the requisition against the listed occupation before anything is presented. The condition is unchanged by the departure: the applicant must be a Canadian or Mexican citizen engaged in prearranged professional activity in a listed USMCA occupation, holding the qualification that occupation specifies. Hypothetical example: a reinsurance group hires an Economist to model loss development; the successor’s draft calls the role a data analyst, and the requisition is what restores the original title.