That doesn't sound right. Perhaps the mil didn't pay any of their registrations or a mistake in the registration office. What did the registration department say when you brought it up.
[§249-6.5] Exemption for National Guard, military reserves, and other active duty military personnel claiming Hawaii as their residence of record. One noncommercial motor vehicle registered to a member of the National Guard, military reserve, or armed service, including the Coast Guard, who is:
(1) Assigned to a unit in the State; and
(2) A member in good standing,
shall be exempt from the vehicle weight tax provided for in this chapter. [L 2008, c 141, §1]