The Frostmourne issue has been reported and is a Known issue:
While Kazakus is listed in the Neutral section it is a tri-class card usable by Mage, Priest or Warlock only. This is shown when you hover over the card in the collection
Ensure to search first before posting to reduce duplicate issue threads. Also in future, one issue per thread.
https://hearthstone.gamepedia.com/Kazakus