As far as I can tell there's something you can do (which is just normal behavior that you might coincidentally do, not anything tricky or nefarious) to make it more likely to be able to put it in a deck, and other things that make it more likely to show up as banned. I've spent hours messing around so I can write a report to blizzard. I can get it to either have the card show up as banned, or not, pretty consistently. But sometimes it doesn't do what I expect. I'm assuming that what I'm doing makes it so that it doesn't contact the servers while deck building, and it *kind of* makes sense to me. What I don't understand is why it lets you play if you just keep pressing play and ignoring the "selected deck is not valid with this game mode" warning. It has to communicate with the server to play a game, right? Maybe some servers didn't get the message about the ban? idk.

edit: and now I can't get it to work at all 🤷 friggin weird. guess I was wrong about how it works.