Description
While the new world generation replicates the surface generation of 1.12.2, the underground generation of caves and ravines is markedly different for the same seed. See enclosed images taken in 1.12.2 and 18w07b generated worlds, taken for the same seed, vantage point and viewing direction. (The 18w07b screenshot is also affected by vines generating at y=64 in jungles, a bug reported elsewhere)

In addition the location of generated structures (abandoned mineshafts, jungle temples, dungeons, etc) differs between 18w07b and 1.12.2. A mineshaft can be seen at the right side of the 18w07b screenshot, not in the 1.12.2 screenshot.

Replication

1. Generate worlds in creative mode in 1.12.2 and 18w07b using seed 4747156497960257822
2. Teleport to 136 125 312
3. Change to spectator mode
4. Apply nightvision to player
5. Teleport to 136 60 312
6. Compare cave/ravine generation between versions

Additional observations:

• 18w07b has an abandoned mineshaft in this area, absent in 1.12.2
• 1.12.2 has a jungle temple above ground, missing in 18w07b
• Tree generation differs markedly