A similar bug was reported by someone in snapshot 18w15a and says that it was resolved, but it seems it has come back in 1.13-pre8 and some earlier pre-releases. When a tree (all variants), giant mushroom, or small End island generates near the edge of a chunk, and part of it is supposed to generate in another chunk, sometimes that part of the tree, giant mushroom, or small end island is not generated. When these structures generate in this way, they can appear cut in half and unnatural. It's possible that it also effects small ponds and lava pools that generate in the Overworld and Nether, and the square patches of land in snowy biomes that generate without any snow or tall grass. As an example of this bug, I took screenshots of a tree, a giant mushroom, and an End island that were generated incorrectly, on a 1.13-pre8 world with the seed -2057924093. Here are the locations where the screenshots were taken; Trees at (-152, 75, -1654), Giant Mushroom at (164, 74, -1565), and the small End island at (879, 56, -862). To recreate this bug create a world in 1.13-pre8 that will generate with trees, giant mushrooms, or small End islands and look for ones that look as if they have been cut in half or have sections missing.

This bug was reported as being resolved in 1.13-pre10 but I've added a screenshot of it in that version also.