The locations and shapes of the amethyst geodes, lava pools and possibly even other features only depend on the last 32 bits of the 64-bit seed meaning they are the same for many worlds. Even the budded amethyst block positions in the amethyst geodes are the same. 

Steps to reproduce: 

1. Create a world with any seed.
2. Find an amethyst geode or a lava pool and remember its coordinates.
3. Create a world with the seed of the first world but with any integer multiple of 4294967296 (2^32) added to it.
4. Go to the feature's coordinates but in the new world.
5. Compare their locations and their shape.

Observed results:

The features have the same shape and location.

Expected results:

Every element of a world should depend on all 64 bits of the seed.

Notes: A similar glitch also effects carver caves (MCPE-154333), structures (MCPE-154939), the end (MCPE-155137) and the nether (MCPE-154947). Only the surface biomes and noise caves of the overworld aren't effected by such a bug.