Why does my structure spawn on the surface?

I'm working on an infection minecraft mod, and there's a planned featere that when a certain underground block gets infected, a new underground structure will have a tiny chance to spawn at that position. The structure itself is fine, but when I try to spawn it underground, using /place, it always appears on the surface. I think this might be an issue with the command, but I can't spawn it with the "Place NBT Structure" world action block, because it's a jigsaw structure, and each piece is a seperate file. I was planning on using the "Execute command" world action block, but if this is just a quirk of the /place command, that won't work. Any help is much appreciated.

Thanks in advance!

-Arcane