The new biome information format for a chunk has 1024 ints where previously it had 256.

This biome information repeats the same 16 numbers over and over. The biome information for 240 out of 256 blocks in a chunk is not being saved. The numbers do not match up for biomes in 1.14.4 chunks.

I created a new world in 1.14.4 and 1.15 pre-3 using the same seed. Seed is 105. I extracted the biome information out of the save file. Here are the results for the chunk at 0,0.

1.14.4: Forest Hills (18) bordering a Forest (4).

18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18
18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18
18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18
18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18
18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18
18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18
18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18
18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18
18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18
18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18
18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18
18 18 18 18 18 18 18 18 18 18 18 4 4 18 18 18
18 18 18 18 18 18 18 18 18 18 4 4 4 4 4 4
18 18 18 18 18 18 18 18 18 4 4 4 4 4 4 4
18 18 18 18 18 18 18 18 18 4 4 4 4 4 4 4
18 18 18 18 18 18 18 18 4 4 4 4 4 4 4 4

1.15 pre-3: Same on every line. I can't even work out what's going on here.

18 18 18 18 18 18 18 18 18 18 18 18 18 18 4 4
18 18 18 18 18 18 18 18 18 18 18 18 18 18 4 4
18 18 18 18 18 18 18 18 18 18 18 18 18 18 4 4
18 18 18 18 18 18 18 18 18 18 18 18 18 18 4 4
18 18 18 18 18 18 18 18 18 18 18 18 18 18 4 4
18 18 18 18 18 18 18 18 18 18 18 18 18 18 4 4
18 18 18 18 18 18 18 18 18 18 18 18 18 18 4 4
18 18 18 18 18 18 18 18 18 18 18 18 18 18 4 4
18 18 18 18 18 18 18 18 18 18 18 18 18 18 4 4
18 18 18 18 18 18 18 18 18 18 18 18 18 18 4 4
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18 18 18 18 18 18 18 18 18 18 18 18 18 18 4 4
18 18 18 18 18 18 18 18 18 18 18 18 18 18 4 4
18 18 18 18 18 18 18 18 18 18 18 18 18 18 4 4
18 18 18 18 18 18 18 18 18 18 18 18 18 18 4 4
18 18 18 18 18 18 18 18 18 18 18 18 18 18 4 4
18 18 18 18 18 18 18 18 18 18 18 18 18 18 4 4
18 18 18 18 18 18 18 18 18 18 18 18 18 18 4 4