Very closely relates to MC-250445.

The bug

Minecraft supports full 64-bit seeds. However, the Nether's generation largely ignores the top 16 bits of the seed, meaning that there are only 2^48 unique nethers in terms of terrain and structure placement; very minor things can still seemingly vary, but not to anything near the expected extent.

How to reproduce

This can be seen in any two seeds that differ by an integer multiple of 281474976710656 (2^48).

1. Create a world with the seed 1
2. /execute in minecraft:the_nether run tp -266 46 -411
3. Create a world with the seed 281474976710657
4. /execute in minecraft:the_nether run tp -266 46 -411

Expected results

As these are completely different seeds, the terrain should also be completely different, even at the same position.

Actual results

The two seeds are almost a complete match, with seemingly the only differences being with configured features.

How to fix

All aspects of Nether generation that only use 48 bits should be made to respect the full 64 bits and produce results accordingly in the same way the Overworld does.

Further notes

When this is fixed, world blending should be enabled for the Nether as to avoid abrupt cutoffs with terrain generation changing.