I'm using the latest version of forge 1.12.2 using a modpack of my own creation.
Every time I generate the world with all mods installed, it is an endless void in all directions. Only modded structures still spawn in.
If I use half or a quarter of the mods, it works. There is some conflict here and I can't seem to find it no matter how hard I try.
I have never encountered a similar issue before, and I have failed to find anyone on the internet who has either.
I went to reddit and discord first, but neither had any idea what they were talking about, or how to fix it, so here I am.
I would say, this is an mod compability issue. So your task now is to take out maybe like 4 mods at a time, and look if its working then. If not restore them and repeat the same on others. I would personaly start with World Generation mods (or those who change environment) like "Biomes you go", "Advend of Ascension (Nevermine)", all the Dungeons mods, and so on.
Creating a own Modpack always asks for troubleshooting so this is something that happens. The mod deleting method will take some time but its working allmost every time. Just have patience =)