I created a random superflat world, then found this. When tried to re-create this world. "It" wasn't at this place Seed 2765716105643356474
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Can confirm for a previous existing world, updating to 18w47a made solid blocks in some chunks decaying like if they were leaves, being replaced by vines at some places.

hey so it said my issue duplicated this one, but they're not related, so I have no clue how I'm meant to report an issue when some idiot, probably a bot, waves it off as a duplication of something it's not related to
It was not marked by a bot, you can check your ticket history tab. And actually in MC-139852, you said you found some dirt or farmland isolated pattern looking like villager farms? But it sounds like the blocks just spawned there like this current issue describes it. Now try to generate the same world with 18w47b and see if the issue is still there.