In text description of R&N doesn’t say 50% “of the current” or “remaining” mana, and they don’t steal all the mana if enemies’ one is at 50%.
With the same text description, Leonidas reduces all the mana if the enemy has the 40% indeed.
I suppose it’s not a bug exactly, as R&N works this way since the beggining, but i think it should be correctly explained in the card.