It's definitely being caused by some of the stews, but the effect on the villagers' skills appears to be random. It happens only to some villagers, and not every time.

Yes, it's totally random! I think that the stew with the "black pepper vine" does it every time but not for all the villagers! It's just "a little bit bad stew"! (except if the villager eat it several time!)

Yes, and made one master to trainee!

It would be just like Arthur to put something like that into the game. The villagers sometimes have to pay a small price to get such an improvement in their health.

You think he did it deliberately!?!
Anyway this stew don't "get such an improvement in their health."
It gets a litle, litle improvement in health and decrease a lot in skills!! So, my advice is "Don't bother with that stew"!!

Arthur does like to include some decisions in his games that have both a good side and a bad side, and this would fit with his style of game development. I haven't asked him, however, so I can't say for sure (and he probably wouldn't tell, anyway).

maybe another stew wont get less skills

Oh yes!! We know that Arthur loves to make little mysteries!!