Where did my skin go?

I logged in the other day, and apparently I no longer own the Dragon Trainer Tristana skin. I got it in a Mystery Gift during the holidays from someone, and now for some reason it isn't in my account. I don't know if the person who gave me the skin getting banned would have anything to do with it, but I checked and the person who gave me the gift still seems to play the game, or at least their last login was a few weeks ago. Why would this sort of thing happen? I have a screenshot of the gifting history where I was gifted the skin, but for some reason, even though the store says I have the skin, the client doesn't let me choose it, and I can still buy it in the shop.