The bug

When a world is loaded in a newer version that has changed the biome generation, and a chunk has the status "biomes" with old biomes saved therein, the chunk with the old saved terrain has the correct decorations for the old biome in the middle of the chunk but not at the chunk borders. The terrain at the edge of the chunk ends up with the correct biome (the saved biome) and mismatching terrain decorations (the biome that would have generated had the chunk been empty instead of partially generated).

Seed1.zip is a 1.15.2 world save where this issue can be reproduced. Using an NBT editor, four chunks in a 2×2 square have been reset to the "biomes" status. These chunks are located in region 0,0 and chunk coordinates 22,24; 22,25; 23,24 and 23,25. The biomes in these chunks is minecraft:nether_wastes (biome id: 8) because this is a 1.15.2 world save. When the world loads in 20w21a, the player will be in the Nether in the vicinity of a nether portal. To reproduce the issue, go into spectator mode and fly towards 368,400 where the chunks are. The borders of these chunks will have crimson nylium and crimson fungus blocks. This should not occur because the world save has no crimson forest biome here. 2020-05-23_14.05.55.png is a screenshot of this location after loading it in 20w21a.