I just got the same bug. I'm so not looking towards replaying those Legendary scenarios, but I'll attempt OP's solution.
Still, it would be nice to get some feedback from a dev on this.
So, I can confirm I got Orik's Shield. If this bug happens to you, here's what you can look for:
- go on the Adventure screen (where all the Adventure Decks are depicted as locations on a map
- select the Adventure for which your Legendary difficulty Treasure didn't unlock
- start browsing the Adventure's scenarios in the list on the left
- you will probably find that on one or more scenarios - despite the fact that they're check-marked as completed on Legendary difficulty- , when selected, their default difficulty icon (the one with the skull) will not be red (Legendary) but blue (Normal) or green (Heroic)
- it is most likely these scenarios that are bugging your unlock
- replay these scenarios on Legendary (despite their default difficulty being set to something else, the fact that they're check-marked on the list should mean Legendary difficulty is unlocked, so you won't have to replay the lower difficulties)
In another topic, a player complained about his Samisen not unlocking, so maybe it's a similar issue, and the steps above will probably fix any missing Legendary unlocks, not only Orik's Shield.