When updating to 1.15.2, if you've loaded an area in a previous version, the exact block to block location of biomes changes. The attached images (1.14, 1.15) are of the change in effect. I re-created the world this happened on in 1.14.4 and updated to 1.15.2 to verify and take screenshots. To more clearly see the effect, I created a flat area of grass above the Mushroom Fields biome, turned off biome blend and placed a fence along the edge in 1.14.