shy echo
So, why are Terralith and BoP different when removing? It doesn't actually have much to do with the blocks themselves. Instead, it's how it works behind the scenes. I am not familiar at all with modded worldgen, so I can't speak on exactly how it works. However, Terralith is a datapack at heart, and therefore uses the Vanilla system.
When you generate the world, Terralith biomes are placed, and the game references the datapack to figure out what to do with the biome. What mobs should spawn there, what color the leaves/grass/water are, whether it should rain or not, etc. When you remove Terralith, suddenly the game sees a biome in the world, but it has zero clue what to do with it. After all, the datapack stores the biome information, and without it, the biome simply doesn't exist. So the game throws a fit and crashes.
As I mentioned, I don't know how modded worldgen works, so no clue how they deal with that issue. But we are limited to the Vanilla system, and that is the unfortunate reality
sick flint