The racial ability isn’t working in the Necropolis campaign

Even on the first map, “Misfortune Never Comes Alone,” I noticed that as soon as the racial ability transitions from Necromancy I to Necromancy II, the button becomes disabled and no longer works for the rest of the battle (screenshot attached). I can’t figure out if this is a bug in the game or some clever mechanic of the Necropolis faction that the hero hasn't yet reached