I upgraded a copy of my world to 1.18 pre-release 2 and got the taiga biome in the middle of a ocean. It has swamp biome in one side and warm ocean behind.
So I removed the chunk using the MCA Selector and opened it in 1.18 pre-release 3 and the same thing happened (see in images).
I believe there must be something with the blending systema that's causing this.