I think it's an issue with the rarity of the item. It's showing up as rarity 0 in my inventory, even though it's actually a retired cash shop item. Definitely doesn't seem like the kind of thing quests should be giving out though. Not sure if this was a ticket thing or a thread thing, so I've just locked it in my inventory for now.

I would make a ticket about it. :)

I know you guys are working on changing rarities as big project, but this one doesn't have one at all xD

We're looking into it. Thank you for bringing it to our attention.

Just letting you know the item's rarity has been fixed. Would you happen to know whether I'm allowed to keep it, or should I file a ticket for that?

Chances are you will be able to, but should file a ticket so everything is documented. :)