That's because Japanese holdouts(残留日本兵), who were soldiers and sailors that continued to fight after the surrender of Japan in August 1945. Japanese holdouts either doubted the veracity of the formal surrender, rejected demobilization for ideological reasons, or were simply not aware because communications had been cut off by Allied advances.
Using the logic of Juneteenth, the Second World War in the Pacific could have continued well into the 1990s. Although the last confirmed surrender was Private Teruo Nakamura, a Taiwanese-born soldier who was discovered by the Indonesian Air Force on Morotai, and surrendered to a search patrol on December 18, 1974. Nakamura, who couldn't speak Japanese or Chinese. He was discovered 29 years, 3 months, and 16 days after the Japanese Instrument of Surrender was signed.
Likewise, it can be shown that slavery still exists, which knocks another premise of Juneteenth: that it was the end of slavery. Any luck it will go back to being the virtue signalling non-entity is was before this year.
But, please feel free to have your picnic and parade. Juneteenth is a quaint reminder of the horrors of war.
Some people get cut off from world events.