Definitely seems like an error. According to the patch notes: "Challenges now allow completion of any ritual activity (Crucible, Vanguard, or Gambit) to progress the challenge in sets of three, to earn up to a max of three Extotic[sic] engrams" But the first two steps give legendary pinnacles, and the final step gives a single exotic that auto-decrypts, not an exotic engram. Something is broken here.