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On 27 December 2021, it was announced that Fukunaga would challenge the reigning WBO super-flyweight champion Kazuto Ioka on 31 December 2021, at the Ota City General Gymnasium in Tokyo, Japan. Ioka was originally expected to make his fourth title defense in a title unification bout with the IBF champion Jerwin Ancajas, but the fight was eventually called off due to stricter COVID-19 measures imposed by the Japanese government. On 28 December 2021, Fukunaga vacated the Japanese, OPBF and the WBO Asia Pacific super-flyweight titles. Ioka won the fight by unanimous decision, with scores of 115–113, 116–112 and 118–110. Fukunaga had a strong start to the fight, but Ioka began to take over from the midpoint of the fight onward. On 10 August 2022, Fukunaga announced his retirement from the sport. The day he submitted his retirement notification to the Japan Boxing Commission was the day he made his professional debut nine years previously
