The bug
If you generate a new world with the same seed in the versions 1.12.2, only the overall terrain seems to be the same. Caves generate entirely different. That affects the nether as well, in fact the nether seems to use the same generator as the overworld now.
This is seed 1 looking up from 0 0 0 in 1.12.2
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This is the same seed in 18w06a:
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The reseeding of decorations is different but more coherent after the refactor. Heightmaps should be identical, but everything produced post initial heightmap generation might show variations.
Interesting, thanks for the clarification!
Initial heightmap seems to be the same indeed, some cave entrances or lakes and such might obviously change the height map after the initial terrain was generated, though. 😉