It was a drop from a Mantid, not from a chest.
I've farmed them a lot aswell and have never got one until recently, it really surprised me. Also found a Bladed Shield and a Paper Fan from them aswell.
Their loot table is quite interesting.
Bladed Shields, sure. Never as golds though, and rarely at any rate. But they're not elementalists. I'll need to see a screen to believe they drop Paper Lanterns.