Glitchy World Generation

https://www.minecraft.net/en-us/article/new-world-generation-java-available-testing

Changes in experimental snapshot 6 compared to snapshot 5
Cavier ocean floors! Aquifers under oceans/rivers are more likely to link to the underground. This means you are more likely to find cave openings on ocean floors that actually lead somewhere instead of being cut off. One consequence of this is that caves at y0-40 below oceans are more likely to be flooded. If you dive into an underground lake you might end up inside one of those flooded caves and pop out at the bottom of an ocean.

Thanks for that reference.
Updated, added more pictures.

The imaged you just attached show MC-30607, not what you were reporting. As for the "giant" terrain in them, that's typical windswept savanna generation, and not a bug.

The report you just linked here only shows a few floating blocks in the biome. However, this one is different based on the screenshots attached above.

I do recognize that partial parts of the biome have floating blocks that look like a floating island, but it's the world generation. There will be problems from time to time. The world generation is mainly based on a unique and creative algorithm created by the game developers.

Now, this is a typical generation for the specified biome Windswept Savanna.**
https://static.wikia.nocookie.net/minecraft_gamepedia/images/9/9a/Shattered_Savanna.png/revision/latest?cb=20201108121840
.

This is just "shattered" worldgen, which in 1.18 will happen in more (most?) biomes. Check the "F" value in the F3 screen, if it's low, terrain will be shattered and weird.