When my world was in 1.15 I completed the "Monsters Hunted" and the "Adventuring Time" advancements. Then I updated it, it rememberd the old biomes and mobs so only the new ones remained, the problem is that when I did kill/explre a new mob/biome it doesn't count in the advancements.
Btw, I tried in a world that was created in 1.16 so that it's made in 1.15 is not the problem.