Cave mountains inside the "Portal" in deep dark single biome world

Put the summary of the bug you're having here

What I expected to happen was...:
The "Portal should be all exposed

What actually happened was...:
It was coverd by the cave mountain and a lot of water

Steps to Reproduce:
1. Load up single biome world on the seed: 7490808444288101578
2. Go to the coordinates: -1 -31 129