As stated above,
Not sure if there are other cards as of yet but today I tried to use “Chief Inspector” from The Witchwood series in a duels deck and it simply did not exist. It shows as owned in my collection but not in the available cards for duels when building decks.
The Witchwood set is not included in the current Duels rotation.
https://playhearthstone.com/en-us/news/23663246/21-0-patch-notes
I thought maybe it was a typo when it was under arena but not duels. After seeing it in supposedly new and played decklists it became a bit muddy. Thank you for clarification!