Fabric 4.10
minecraft 1.19.2
MaLilib 0.13.0
Litematcia 12.5

Trying to place a schematic under ground and once you hit Y0 it starts to disappear.

And you are not in a render layer mode such as All Above or Layer Range?

I am not. it just disappears a y0

Would it make a difference if I changed the layer range to -15? because that's where I want it

In the All mode it's definitely supposed to render it all. And it does work in my test world below y = 0. Can you test it in another test world?

It shouldn't matter what world the schematic was made from, only what the world where you want to load it in reports as the world bottom. But if there is terrain below y = 0 then it's strange if it would get the bottom wrong. Especially since the MiniHUD slime chunk overlay you have on is rendering below y = 0, so that one is getting the correct world bottom.

okay, I figured it out, and I think it's a weird world-generation bug with complications. The world that I designed this build in was a 1.15 Super flat when it was generated. Which of course the world ended in Y0. I took this Litematic Schematic and put it in a new super flat which is at -55 or something and it wouldn't render. I used world edit to make a WE copy and loaded it in the new 1.19 super flat, and then make a Litematica of it, and it works. I've just never designed a build below intended below Y0 before so I'd never seen it.

I'm assuming it has something, with old super flats that have been upgraded several times, has some rare weird glitch. Which means it's going to be next to impossible to track down. I wish I had something else for you to go on. But it's clear that this issue with due to this world and not Litematica.

I thought it was strange too, which is why my only reasonable explanation is my super flat has some world corruption in the level.dat