I noticed this in a few of my games too. some of the skins we were able to play were dragon slayer braum, riot k-9 nasus, and a few others. we found out its when you trade champions in dynamic queue. 'ike say for example you were with a friend, they were top floor, you were bottom floor, if they picked you nasus, and you picked him braum, and then TRADED, i don't know if you have to click done, or choose a skin, but that was common both the times that i was with a friend after trading they said, hey i don't have this skin, but cool! haha
ALSO MAYBE A DIFFERENT BUG:(maybe the same) whenever i would get nasus, or choose nasus, it would already have chosen the infernal nasus skin for me. i pay archduke because on infernal nasus, there is a little bit of a delay on the animation of his Q. but EVERY TIMEi chose him, i had to SWITCH IT from infernal to archduke. same thing happens with leona, i play project leona and pool party leona, but every time i choose her, it doesn't default to the last one i played, it always defaults to the classic skin.
Lastly, THERE IS NO WAY TO CHANGE YOUR WARD SKIN IN THE NEW CHAMP SELECT. i like to synergize my skins with ward skins, like playing underworld wukong with the underworld poro skin. I can only ick it in draft, or in the store. but i cant change it in the new champ select. thanks riot!