If you use the Elementium Shears on some sheep while you don't have a mana tablet on you it takes damage as it should, but it doesn't repair that damage when you pick up a tablet with mana up later. Given that the armors do repair themselves with mana later I'm guessing that this isn't the intended behavior. This is with the latest (122) version of Botania.

This is intended, yes. The armor repairing is because of Thorns.