There's a bug with the current cave generation where a cave could split, creating a 1-3 block thick wall in the middle of the cave. This bug is a lot more noticeable when generating less caves.

Here's a few example of this occurring.

Edit: Changed the second image because it was at the edge of a cave

What I meant about the second comment was them when I was generating a world with less caves then default, I noticed them a bit more quicker than if i looked in default.

Edit: Now that I think about it, its a bit hard to confirm that this bug is valid. Maybe we should brush it aside and focus on other bugs until there's an easier way to prove or disprove the validity of this bug.

Isn't this normal vanilla behaviour, but more noticeable because of the low amount of caves?