I am using the latest Biomes o Plenty for 1.16.3+: Biomes O' Plenty 1.16.4-13.0.0.426

I install the mod and go into the nether portal.

It spawns me in what is once known as a Crystalline Chasm biome.

I notice weird spawns from the overworld. Zombies, Spiders and Skeletons, Oh my!

Look at mini map and f3 and it says "Tundra".

I do the /locatebiome and Crystalline Chasm says it is there, but the game says it's a Tundra.

Serene Seasons could be affecting this?

Some Screenshots:

If you need more information tell me. I can also tell you all the mods I have.

Please do tell us the mods you have. In all likelihood it'll be something like Optifine corrupting the biomes in your world (which was only fixed recently).

It's a Custom modpack mod files below pack file: https://www.curseforge.com/minecraft/modpacks/geosmelt-1-16-4/files/3155810

Mods would take forever to list, sorry.

I will do a test with just Biomes and Serene Seasons then go from there.

I will note that regardless of what you do now, your world is permanently broken. This has happened because at some point the biomes have been corrupted. Usually old versions of Optifine are the cause of this. The original generation will remain, but the biomes associated will not be correct. Your only option is to find a backup from before this happened.

Another potential way this may have happened is if you upgraded from Biomes O' Plenty for 1.16.3 to 1.16.4 on an existing world. They aren't compatible as a result of the biome rework in 1.16.4.

Don't think I had optifine in the pack or even personally used in about a year, so I know it can't be that.
This world was also created a few days ago, its really just my "test my modpack" type of world.

I wonder if BetterEnd forge is doing it? That mod is still a wip. Yeah world is corrupted because it works normally.

Perhaps? Beethoven92/BetterEndForge#3

Not really sure which mod exactly would be causing it out of that mod list, though I'm reasonably confident it wouldn't be an issue with BoP itself. I had a look at the modpack and it appears they have been using up-to-date versions of BoP for the past few versions so I'm assuming that upgrading from 1.16.3 to 1.16.4 BoP isn't the issue.