structure generation

Huh. You could try doing a world feature that places a structure instead, (I've been able to get these below y=0 in the past), but that is strange. If this is genuinely an issue, might be worth filing a bug report.

If you're using an older version of MCreator, you just make a structure like normal. In later versions, (after 2023.4), structures and world features are separate things. The idea would be to make a world feature that runs a procedure once its placed. (Structures are added from the world seed, meaning you can't really do this using the normal system.) 

Structures generate before/during the rest of the world generation; their positions are predetermined before the world generates. (That's how /locate works on them.) World features are added after the rest of the world has generated, (trees, boulders, etc), meaning they can also run procedures when you generate them. (Some of the older minecraft structures, such as dungeons, are actually world features.) In older versions of MCreator, the 'structure' element  is actually for world features; they only added real structures after 2023.4, but they're arguably less useful.

The basic idea is that you want to make a world feature that, when it generates in the world, runs a procedure. (Something that checks if the surrounding terrain is okay, randomly places a structure, modifies said structure, whatever you're trying to do.)