If they're set to Rest and still taking some damage it's probably not the job that's doing it. It sounds like it might be Toxicity, which is built up by using transformation potions and wears off slowly over time. I think it wears off faster if the slave is set to rest, but it can still take a while to go down.
Once Toxicity gets over ~25-30 a character can expect to take some damage every day, and it doesn't take that many potions to get it up that high. If a character has taken Toxicity damage it should be noted in the daily log somewhere. You can also check their specific Toxicity level with the Mind Read spell.