How to get this to occur:
Put an item with multiple rarities in a trade (I used lowrider wheels).
Then overwrite it with anything else.
The rarity dropdown menu is still available and you can select any rarity you previously could for the other item.
When publishing that trade, the bugged item shows its normal rarity as intended, but it specifies it behind the name: https://i.imgur.com/mQyvWSa.png
On the App, you can actually see the changed rarity, but it doesn't affect how the item behaves then interacted with: