I thought I'd circle back and post the results of cancelling my mileage tickets due to an inability to get into Japan. ANA did make the expiration for my refunded miles three years from the date of refund: October 2025. FWIW, I cancelled in September (prior to an announcement of the opening to tourists) and miles refunded earlier this month. That said, with the country being re-opened, I'm not sure if my situation is applicable going forward.