I saw that now very often in many versions from 1.14 up to 1.16. Sometimes trees and mushrooms are generated cut in half. First I saw that in the old Pocket Edition, then around the start of 2018 in the official game. But the Bug was solved once. Now it is back again.
I saw that in 1.16, but also on the Bedrock Edition. I thing it has something to do, with the rendering distance, because it mostly happens, if Im walking around, leave the world, the join it again, and go to the borders of the render distance that I saw before I left the world. But I saw that already without that. It goes like a straight line along one specific coordinate where all trees are cutted (like on my screenshots)

Steps to Reproduce:
1. I generated a new world with large biomes, but it happens in a normal world too
2. just go or fly around in It and Watch out for trees and mushrooms.