After playing in a world for a while, weird wall-like generations appear in chunks I have already explored. These generations were not previously there, and randomly appear any time I revisit a chunk. I have had friends enter the world, however they are not player-created, as I find floating sand and sugar cane built within them (refer to the pair of images with sugar cane, where I found a new generation with sugar cane drops; change to “Sort By Date” next to attachments to see comparison). I do not have cheats on either.

The images attached below show 4 different areas: the top shows the generations found in one of my survival worlds I created about a month ago; the bottom shows what the terrain should look like, taken from a creative world I made earlier today using the same world seed.