After defeating an ancient mooshroom, they dropped gilded Gauntlets with typical gauntlet features such as the icon and lacking any unique perks, but have the label of "unique".

Luck of the Sea was in effect at the time though it's unclear if it, or the fact this was gilded/an ancient hunt drop, is related, though this is the first time I've seen this. It did this behavior once more the very next time a mooshroom ancient dropped gauntlets, but the time after that it dropped proper Fighter's Bindings, all within the same session, so whatever happens doesn't appear to stick for long.

It seems possible Luck of the Sea mistakenly forced the Unique label alone onto a rare item rather than fetching one of the unique variants.