I was doing an Ancient Hunt run with an armor enchanted with Luck of The Sea enchantment as it says "Increases your luck, which boosts the chance that rare drops become unique." One ancient mob happened to drop a Thief Armor. But, instead of dropping its Unique variant, which is the Spider Armor, the ancient mob drops a Thief Armor with UNIQUE flair on it (notice the orange glow too). I'm not sure what causes this bug. But it was my first run of Ancient Hunt using an armor enchanted with Luck of The Sea, and at the same time, it was my first run using the Luck of The Sea enchantment. So my best guess is that it has something to do with the enchantment.

And as for now, the bug has only occurred a single time (since I've only completed a run with the enchantment so far). I don't know if this bug could occur outside Ancient Hunt, or for other gears in the game though.

Anyway, I'll provide an update if the bug reoccurs.