Unfortunately, I do not have any experience with this matter. It does seem like a major oversight on part of Liberty Mutual. The only thing I could suggest would be to speak to a manager at Liberty Mutual or seek legal advice. Bear in mind, the fees you occur from small claims court and an attorney might be greater than the refunded premium. If you're unhappy with the experience you have received, I recommend shopping around. You can find different car insurance companies in NY with customer service recommendations here.
Best of luck.