How do I stop structures from spawning above the ground?

I'm trying to make structures that are basically just coral from coral reefs but dead. They're set to spawn on the ground and set to only spawn on stone, but I've spotted a lot of them spawning in the air. They're not just spawning 1 block in the air either, they're all spawning at different distances between the ground and themselves. The lowest I've seen was actually in the ground and the highest I've seen was approximately 100 blocks above the ground (the biome they spawn in has high height variation).

How do I stop this from happening? I thought of just including some stone under them in the structure, but the height above the ground is too unpredictable. For now I guess I'll just let them spawn in the air and move on to working on something else, but I'd love if someone could help me figure this out. I've tried everything and none of it worked.