I few days ago I unlocked the achievement: "Dancing Rave Style You danced so well with your chosen partner, you got an achievement from it!" but I did not get the related cosmetic border from it. Is it bugged? Does it take multiple days to unlock? Will it only be unlock at the end of the masquerade? I was kinda hoping to equip this border since it fits really well with my current masquerade avatar :/

Border not unlocking is a known issue that many people are having affecting both of the achievements (Rave & Steamwork)

Bug Fourm Topic: HERE

I imagined as much, but it is nice to have confirmation, thank you very much for your answer!