I completed the normal pleasure gardens challenge earlier in the week and it counted towards the ornament, but when I completed it on prestige this afternoon (got the second chest and drop) it didn't count. Went to Benedict and it still shows I need to complete the pleasure gardens prestige challenge.
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Bungie has confirmed the ornament descriptions are bugged. They’re for the new raid lair gear, and they’re unlocked by doing prestige and challenges in the new raid lair.