I have enabled the "Caves and Cliffs" experimental features on a world, and occasionally, blocks are being added/replaced where the world has already been generated. This is definitely down to the new world generation introduced in 1.17.40 - before this update this issue did not occur even though I had "Caves and Cliffs" enabled. The most visually obvious issue is that lines of leaves (about 5ish blocks long and sometimes two or three parallel to each other) and individual blocks of vines are being added at random points in the sky, replacing air blocks. These will not go away by themselves despite there being no log blocks surrounding them. Stone blocks are being replaced with other cave blocks (such as ore, granite, andesite etc...). Grass blocks are being replaced with podzol. Decorative blocks (ferns/long grass/flowers/etc) are being added on top of grass.

The issue is not disruptive to movement or line of sight; the replacements are in small clumps and scattered far from each other. But they are still severely disruptive to the general experience. When I "fix" them, my efforts are in vain as new modifications to the world are made someplace else. Adjusting random tick speed appears to have no effect on the frequency/speed at which new changes occur. I have witnessed these changes occuring inside the player's rendered view, with only one or two blocks between them.

The world was originally created in the PS4 Legacy edition in mid-2015, although I don't know if this makes a difference; I have not tried more recent worlds with the same settings.