In 1.17.1 the were 42 biomes needed to earn this advancement, but as for 21w40a it has downgraded to only 31 biomes needed. Since there're new biomes like meadows it should have increased instead of showing less biomes now.
Comments 2
Like @unknown said this is intended. Biomes were removed in 21w40a leading to less biomes to be explored. The issue with the mountains is MC-236701
Pretty sure this is WAI since a lot of biomes have been consolidated for 1.18.