Worlds created in 1.6 have taiga biomes that it used to snow in. These biomes were converted from snowy taiga to regular taiga in the 1.7 update and it no longer snows there. When blending these chunks in 1.18, the blending does not check if these chunks are snowy and so the snow does not get blended. You end up with the biome blending perfectly fine but you get harsh edges to the snow where it ends.

What I expected to happen was...:
The blending should check for surface snow and add snow into the new chunk to smooth out the edge of these old taiga biomes.

What actually happened was...:
I ended up with ugly harsh edges to my snowy taiga from 1.6 that was updated to 1.18.

Steps to Reproduce:
1. Generate a snowy taiga biome in 1.6.4 or bellow.
2. Upgrade to 1.7-1.17. The biome will now be called just "taiga". (alternatively skip to 1.18, the results are the same, but in 1.7, the snowy taiga became its own biome, so these snowy taigas stopped being snowy and it started to rain there instead.)
3. Upgrade to 1.18 and try to generate a new chunk next to this old snowy biome. The blending will not work with the snow. You will get more taiga biome, but not the snowy variant. There will be ugly harsh lines, exactly what the blending feature is aiming to remedy.