On January 26, 2018, the U.S. Department of State revised the Foreign Affairs Manual provisions that guide visa officers on the validity period of visa stamps and the number of admissions to grant under a visa stamp.
Under this new policy, visa officers may limit the validity period of visa stamps to a time frame less than that required under the government's reciprocity schedule if the officer believes the "stability of the applicant's longer-term ties to his or her residence abroad are in doubt." Officers may also reduce the number of admissions/entries typically permitted under a visa stamp. The new policy does advise visa officers to use these limitations "judiciously" and "with caution", and only if an officer has "due cause." More information and an example of a case that may warrant a visa validity reduction are provided at 9 FAM 403.9.
It is difficult to predict what, if any, implication this new policy may have on the issuance of TN visa stamps by U.S. consulates to citizens of Mexico. The reciprocity schedule already only provides a 1-year validity period to TN visa stamps, and I would suspect that the reduction of this validity period would be reserved for only a small number of borderline cases. Should this be otherwise, I will provide an update here.