I have a superflat world i made before Caves and Cliffs that i use for just general goofing around, and after I updated my world to extend the world height and depth, a weird bug has been occurring where if i let water drop down to the very bottom of the world, even if i get rid of it, the water will reappear the next time i load up my world again. If i place blocks down there to try and get rid of the water, the water will stop reappearing but now the blocks keep popping back up whenever i load the world up. This means i have to create designated spots where i have to keep floating blocks in the void forever because they refuse to stop reappearing and it creates such an ugly effect whenever you can see them, and it really bugs me.

A similar issue happened when i updated to the wild update where a few builds i had intentionally built below the old world limit lost entire massive chunks of blocks, forcing me to replace them all by hand, which even in creative mode was just a tedious and cumbersome task that emotionally drained me, and one build i had to entirely scrap because it took advantage of the new world depth to create a reflection effect, which the bug caused the entire reflected portion of the build to vanish, and i was so distraught, as well as dreading the idea of having to rebuild the entire room upside down again, i just scrapped the entire build.