I had the same issue today with Adventure 3, The Hook Mountain Massacre. I completed all the scenarios for the "Black Arrow Longbow", it shows you the item on the reward screen, but then it doesn't show up in your stash, your unclaimed cards, or any of the character's decks.
Normally when you get loot at the end of a battle it will show up in one of the character's decks and you will have to remove down to the maximum. Is it possible there is a glitch with the decks or the stash being full which doesn't let you receive the card?
I would definitely love it if all that effort wasn't a waste. I am going to try and beat another legendary level in that adventure to see what comes of it.
Using an iphone 7
game version: 973-20170628
PFID-48BA8AC38B144CB3