Looking at a biome map in 1.18, I saw that some biome borders are too smooth. The biome borders incriminated are (at least) transitions between different levels of temperature and/or "dryness".

You can see attached the map clearly showing those smooth borders. I have also uploaded the same biome map where I highlighted some of these very smooth borders. There is also a screenshot ingame of such a smooth transition (same seed).

I think that the usage of a fractal noise function would mostly solve the issue by making borders rougher at any level of details.