Playing in a survival world, I come across a snowy mountains biome in 21w39a, but for some reason the game doesn't think it's there and instead thinks it is a snowy tundra (which it's not). There is a snowy taiga directly bordering it at the top, but a regular snowy taiga surely doesn't generate at y=120+. This weird patch of land has the exact same traits as a Snowy Mountains, but the game says there's no nearby tall/mountain biomes at all (including Snowy Mountains).

To reproduce

World Version: Snapshot 21w39a

Seed: -7923636019623933797

Coordinates: [-944 ~ 905]

Open the F3 menu, and you'll see that the game says the Snowy Mountains is a Snowy Tundra, and /locatebiome does not say there's a snowy mountains within the search radius.