This issue was listed in the most recent round of hotfixes, but still appears to be bugged in-game. It's possibly the issue was only corrected for new characters completing the quest.
The optional objective is hard to miss in front of the area's exit portal.
The quest itself is quite simple, a rehash of the Behind Enemy Portals world quest from Legion. Transforming into a Wrymtongue to avoid demonic forces, players are directed to loot the area, by quite literally collecting treasure chests scattered around the area. The optional objective is very obviously played at the end of the miniature maze and hard to miss, which players would want to collect before they are prompted to return to the Arcantina.
However, upon returning and turning in the quest, there's no place to display your recovered treasure. The prevailing theory is that because Inactive Filigree Moon Lamp is uniquely tagged as a quest item, the game just discards it after turn in, but whatever the case may be, Blizzard will need to solve it before players check that requirement off of the Highly Decorated achievement.
At least you're still treated to a heart to heart between the Warlock Tehd Shoemaker and his unlikely Demon Hunter partner, Marius Felbane.
Like most Arcantina objectives, the quest can be completed very quickly, but does not provide a return portal. With the Personal Key to the Arcantina now on a 15-minute cooldown, this means you could get stuck there for awhile if your hearth isn't in a convenient location! There's another entrance at the Silvermoon Inn, so consider using that to pick up the initial quest and using the Key to get back instead.
Remember too that a new Arcantina quest is offered each week, though they do not need to be completed during that week, so consider waiting to turn in Still Behind Enemy Portals until the issue has been fixed.
