Bug Report

[ Crystalline chasm Biome meant for the nether is in my overworld after going to the nether for the first time]

What's the issue you encountered?

[ So I just started a new game with some mods, biomesoplenty and terraforged alpha included (I always loved that combination for world gen). Found a village, built a wall around, plains biome, all normal. Built a nether portal on the surface in the village, went in, found some stuff, got back, sleep, some mining, then when I went near the portal again I noticed some particles, like shiny particles in the air, then the ambient music and sound changed, then a purple fog. I pressed F3 and the biome said that I was in a crystalline cham biome, in the overworld, and is not just 4 blocks around the portal, is a large biome area.]

Logs

[ Idk which logs would be useful since I don't even know when this started to happen.
modlist.txt
]

Mod Version

[ BiomesOPlenty-1.16.4-13.0.0.425-universal.jar]

This definitely doesn't happen with BOP alone. You're going to have to start with BOP only and go through your mods one-by-one to find the culprit. It's most likely Terraforged though, as that takes over overworld generation.