The loot box you get once you reach level 5 question

So, when you level a new account, and you reach level 5 you get a loot box. All my other accounts, when you open that box you would get 1 skin for a champion you own. When did they change it and just gave you a random skin? I've done it before on all of my alts and this is the first time it's on a champion I don't have. (Please Note...I own ONE champion on each of my accounts...I got one champion specifically to get a skin for that one champion...)

Edit: no on knows if it's been changed?