I have owned the Cupcake card back since it’s inception back in 2015. I have always used it for all of my decks. I took a short hiatus and when I returned I noticed it no longer showed in-game. It shows the original/stock hearthstone card back. I went my decks and made sure it was equipped and it was. I then went and made sure it was my favorite and it was. I tried making another card my favorite and re-favoriting the cupcake one but it just doesn’t favorite for some reason.
I also noticed this with the Naxx card back as well. Since the cupcake card back is supposed to go on sale and the Naxx card back was on sale recently might this be causing an issue to those who already owned those card backs? I went through the troubleshooting process a gm asked me to go through and have even re-installed the game a few times to no avail.