I might understand more then you understand what the “unique” trait on an item means. No, go look at it again, it’s one of the VERY few items that is “unique”. Its right underneath “binds when picked up”. This is not “unique equipped” so it literally means “you can ONLY own ONE of these in your possession at the same time”. Not 1 on your person and 1 and your bag.
Is it a bug? Very unlikely since it’s been this way for years. This is the first time anyone’s actually cared though. Is it dumb? Yes. I’d gamble that now enough people will complain so they might make it “unique equipped” finally.