Error in revising existing worlds.

I have a survival world that I've been playing in for almost 4 years. I experimentally set it to Caves and Clives in 1.17.40.

Especially near my base I can now see scattered blocks of leaves and blocks of water
floating in the air. They look like the remains of a high line of mountains. As if mountains were briefly added to the world generation, then removed again, just not completely.

my seed is 1260256863.