The bug

When a custom structure generates with underground or uses jigsaw blocks to generate underground, the custom structure will not generate any new pieces under Y=0, it will still generate pieces under Y=0 just not any new pieces. For an example: in the first picture, the structure is cutoff below Y=0 and is not generating, resulting in a cutoff in the deepslate layer.

I have attached a data pack that can be used to reproduce this issue below.

How to reproduce

1. Download the data pack provided
2. Create a new world with the seed -8285837402283326843
3. Use /locate ms:catacombs and teleport to the location provided
4. Go to the bottom floor of the structure and notice how no bottom paths are generated under Y=0 resulting in a cutoff effect.