this issue has been going on for a couple versions now. I'm using 0.5.17b, and my slime has 100 toxicity and is constantly mutating. I pulled open the code, and it looks like the calculation is messed up.Doesn't solve the underlying issue that toxicity is borked at the moment, even if you only use humans. You haven't noticed that no matter how many potions you force your slaves to chug, their toxicity remains at 0?
Whenever you would gain toxicity, you instead lose it, and vice versa. so the Slime's ability that should 0-out her toxicity is setting it to max instead, but giving her potions actually lowers the toxicity by the toxicity's value.