General Structure Placement

Trying to learn more stuff with all of this.
It seems to make the bottom unnecessarily big, or just outright not care about placing land under the structure
I assume this is just a limitation of how this all works and I should probably add some basic terraforming there in-case it ends up floating somehow.
But I figure I might as well ask first.
I attached some image examples and the last one (green one) is an example of where all my structure voids are placed at, basically the same in both
I am using beard_thin for the terrain adaption
Here is a part of the code
{
  "type": "minecraft:jigsaw",
  "biomes": "lnm:forgotten_depths",
  "terrain_adaptation": "beard_thin",
  "max_distance_from_center": 80,
  "size": 6,
  "spawn_overrides": {},
  "start_height": {
    "type": "minecraft:very_biased_to_bottom",
    "min_inclusive": {
      "above_bottom": 38
    },
    "max_inclusive": {
      "above_bottom": 88
    }
  },
  "start_pool": "lnm:random_barrel",
  "step": "surface_structures",
  "use_expansion_hack": false
}

And do note it is just a single .nbt file