Expected behavior after updating to 1.18:

Only biomes at the edge of already generated terrain will have their borders changed due to the blending of old and new terrain.

Observed behavior:

Old terrain (either created in 1.16 or 1.17.1), that is not at the edge of generated terrain, but surrounded by at least 12 already generated chunks, has it's biome borders slightly changed.

How to recreate:

Create a world in 1.16 or 1.17.1 and load terrain with a render distance of 12 chunks, then update the world to 1.18 and the biome borders everywhere will have slightly changed.

Information about the screenshots:

The screenshots are of a world with the seed: WLLBYUG. The coordinates where they where generated are -19 62 -200 for the first two (1.17_1 and 1.18_1) and 36 68 196 for the other two (1.17_2 and 1.18_2).

If the biome borders changing is an intended/unavoidable feature of 1.18 due to the complete overhaul of terrain generation, please let me know.