Overworld biomes register in the Nether

For some reason, after updating my game to 1.18, some of my existing Nether biomes have changed too Overworld biomes (in the code) for some reason, and so I get Overworld hostile mobs spawning in these areas, which makes traversing the Nether ever harder.

Note: I am using mods, and I did report the issue to the relevant creators first, and they said that they don't officially support upgrading worlds and the issues that it brings, and that I should take up the issue with the official MC bug tracker. Even if a change is made to prevent Overworld biomes from being able to generate (in the code) in the Nether, any help would be appreciated

World Type: Custom

Seed: Occuring in at least 2 of my worlds; no specific seeds.

I had Bonus Chests turned on.

 

UPDATE: This issue can be reproduced in vanilla; I have loaded this on the same world on 1.18.2 in vanilla, and this issue occurs in the same locations. I have posted new screenshots below: