Terrain blending of old worlds has sharp boundaries for snow on windswept hills

When upgrading old worlds, the terrain blending has sharp snow borders on the windswept hills.

To reproduce:
1.Create a world in 1.17.1 with seed 3. (This seed has windswept hills near spawn.)
2.Load this world in 1.18-pre1 (or other version being tested).
3.Inspect the biome blending at the edges -> No snow on the windswept hills where the biome was blended.

Likely cause: snow turns to rain at a slightly higher elevation in these biomes.