World generation made a tree and some grass floating above water

What I expected to happen was...:
trees and grass wouldn't be floating in midair

What actually happened was...:
a tree and some grass were floating in midair
The grass disappeared after a while, but the tree remained.

Steps to Reproduce:
The seed is: 8678986397600235714
Coordinates (x, y, z): -71 64 243 facing north