There are two places called the “crevice” from your description you mean the one in the northern biome. And also you seem to be playing single player.
So, I think something is wrong with your game.
Because in my single player game, I have a huge castle built in the crevice, it’s thralls are fine, and have been for years.
I also built in the northern “crevice” on Official servers twice, and my thralls never suffered and adverse effects.