I think it could be related to a similar issue we saw with Death Knight’s recently, with some valid cards being automatically removed from decklists. When I imported the code for the deck above, it did that same thing for acidic swamp ooze, even though the card was still obviously valid.
After trying importing the deck code from two different sites, I built the deck manually without any gold cards, and that is the deck list pasted here, which still did not work, unfortunately.
Unfortunately, it’s STILL broken. and so are some of the older puzzles. and it SEEMS like Blizzard just doesn’t care. if it’s not making them money, they don’t touch on it.
Hopefully Blizzards sees most/all of the “Mysteries” challenges have this issue, and resolve soon.
Until then, a user has kindly provided a little clarity here, though I’m not sure it will help with any solutions:
If a card is currently in Core, is it even possible to build a deck with the Wild version of that card?
In my experience, the deck builder UI prefers to show Core cards when possible.
I really hope the team will look into this. I only started playing Hearthstone a few months ago and I have really enjoyed doing these puzzles. It’s a bummer the Barrens and Nathria ones cannot be completed presently.
To give them the benefit of the doubt for a moment, as large as their team is, they can’t be working on all things at all times. As much as we value these challenges and get annoyed at them being broken (or coded in such a way that they would definitely break in the future 
), I can’t exactly fault Team 5 for not fixing this right away.
I don’t even think it would be difficult to fix, I suspect it is only a few card ID checks that are too specific, however as unfortunate as it is to say, it is unlikely team members will be given the opportunity so long as higher priority work items exist.
I hope they can find the time though.
(post deleted by author)
resolved, w/ context: https://twitter.com/Celestalon/status/1674544679067721728?ref_src=twsrc^google|twcamp^serp|twgr^tweet
I’m in!!! - Here’s the deck code & notes:
Now, if you’re like me, and get confused by the Core v. Legacy sets and what “evergreen” means, exactly, use this decklist below, but pay attention to the “important note” below. It’s like a game within a game now 
IMPORTANT NOTE: Make sure each card is the version FROM ITS SET, not the core version! - The only two core cards are Ysera and Noz. - Every other card should come from its own set. - For example: for Tar Creeper tab to the Un’Goro button on the side to select it, don’t search “Tar Creeper”, or it will default to the “Core” version. - Make sure to click each set name to pick the cards, don’t use search. - If you’re still confused, look for the icon on the back of the card, underneath the game text. “Core” = a wolf icon. Make sure it’s not the wolf icon, and is the icon for the HS set it came from (Except for Ysera and Noz, which have the wolf/core icon).
Happy achievement hunting, again!
AAEBAdjwBAzhBIoHt2zFxwLD6gLL7ALv8QLBiQO0igS2igT82wTL4gQJysMCqNkEtdkE9NwEs90EteIEpeQExe0Eyu0EAAA=
Thank you! This worked - should be pinned to the top of all discussions on these mystery card backs. Well done!
if not pinned, hopefully a lot of users find it through google search! glad it helped 
It worked! Thank you very much bro!
THIS DID NOT WORK. Whatever is wrong it is not working if you read this post DO NOT USE ANY DUST TO CRAFT THIS DECK IT WILL NOT WORK!
I did the shaman challenge prior to the new set being added, and just tested it now, it works fine. The only one that wasn’t working for me was the hunter one which is now fixed.
Is there anyway we can get this done without having to craft wild cards for those that are dust limited. For a lot of people that just got back into the game or are new, crafting Zola and kragwa with no intention of using them afterwards seems like a high barrier of entry to get this.
Yesterday I achieved the secret level after crafting Zola and use all crafted cards (except ysera and nozdormu) but core cards changed today and there is no more Ysera there. Which card should I use to start secret level now?
You’ll have to either craft her or open her from a Wild pack. It usually takes the devs a while to rewrite the code for these after rotation so if it doesn’t work I am certain they will fix it.
I found this ysera in legacy and crafted it. Still no secret level after starting queueing wild🤔
So -4800 dust total 
Good news, Celestalon is aware of the bug and plans to have it fixed.
Hey. Yeah, dang, missed fixing this one in time for the next major patch, sorry about that. Will try to have this fixed in 29.4.
Source: https://www.reddit.com/r/hearthstone/comments/1b654m2/comment/kwkjgh7/?context=3
Celestalon comes through again! Thanks for sharing