I think I get it!!
At least, I get one. It's a stew that the villagers don't eat by their own!! and maybe the red Anthurium is not responsible but the black pepper vine (it's the only stew that makes the villagers healthier and have this herb, I think) After eating this: (spoiler) Red Arthurium-Orage Bromeliade-Black Pepper Vine (on the key board: 3-2-9) (end of spoiler) 4 of 8 villagers had their skills decrease from around 5% less!! And more if they eat it twice (my Master Doctor became an Adept!):(
Maybe you should test it also.