Biome borders change when upgrading a world to 1.15

The bug

When opening a world from 1.14.4 (or earlier) in 1.15 (since 19w36a), the biome orders change. This is probably a side effect of the Minecraft save files now supporting three-dimensional biomes, and as such probably intentional.

This is problematic since it breaks farms that are based on only allowing spawning places in a certain biome.

To reproduce

1. Create a new world with the seed 5611121500148513396 in 1.14.4
2. Run /tp @s -1261.5 131 132.5
3. Turn "Biome Blend" off
4. Look at the biome borders
5. Close the world and open it again in 1.15
   → The biome borders change