I opened the caches with my engram inventory full, the rewards went to the post master and when I went to retrieve them it turned the cache rewards into legendary engrams meaning I couldn't get the Horror Story or Braytech Werewolf! I am very infuriated at this as I wasted my time going through a run only to not even get my hands on the event weapons!
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Legendary Engrams are one of the possible rewards from the Haunted Forest caches.