The bug
Some biome decorations such as snow, trees and structures sometimes fail to generate, producing very distinct 8 × 8 sections of land within a chunk from the surroundings.
Mod edit: Fixed for all but trees as of 1.13-pre10
Something to note is that they appear to be random; after re-creating the world with the same seed and settings, some of these odd chunks are generated correctly, and other chunks which generated correctly before seem to suffer from this bug.
Duplicate photos from the attachments show two versions of the occurrence: two in different worlds with the same seed, but only one showing the glitch.
How to reproduce
Because of the hard-to-reproduce nature of this bug, I can't guarantee the same seeds will produce the bug at specific places every time, but it should be easy to find an occurrence on a large ice plains' sheet of snow or villages' houses and paths.
Just in case
If it's of any help, here's the seed of the world I took the screenshots on:
3239279967284580547
Linked issues
is duplicated by 80
relates to 5
Attachments
Comments 57
causes random ice to spawn outside of cold snowy biomes randomly in the sea. also you get double trees where birch trees part grow in oak trees
Also affects 18w10(a/b/c/d).
[media]