I’ve been having this issue for a few months now: I start up my game, go into my collection, and see cards I already obtained marked as new.
I’ve noticed a pattern and this only happens if I’ve mass disenchanted my collection the last time I closed the game. Then when I launch the game the next day, it shows the cards I had extras of appearing as “new.” Any solutions?
This is a bug that has been present for more than 2 years and in my mind will never go away. Whenever you dust 3 or more copies of a card - without mousing over the card after you disenchant - the game will mark the card as New.
The game is currenly unable to know how you arrived at 2 copies of the card, but it does mark whether you have seen the card. This ‘seen’ check is however not set for duplicate copies you got from packs. So the logic that determines “New” to appear, doesn’t see the ‘seen’ mark on all copies, and sees 1 or 2 copies total. So it assumes you had 0 or 1, and therefore marks the card as new. Again.
Only solution currently is to mouse over them like an OCD.
I understand the cynicism. However, very recently the “rush/charge bug” that was even older was finally fixed, so Blizzard apparently DO care about fixing old bugs.
This does not affect gameplay so it might perhaps be lower priority but I still hope to see it fixed one day.
There is a slightly easier way to do this.
Before you mass disenchant, review the new cards that you have not reviewed yet (because I assume that’s what you want to use the “New” tag for.
Then use mass disenchant, and after that you know that there will be a bunch of “New” tags and they are all fake because you don’t even have a new card. Go to the collection manager and type “New” (without the quotes) in the search field. Now you see only cards with a “New” tag and you can simply wave your mouse over the screen, advance, repeat.
Still something we should not have to do, but a lot faster than trying to find all “fake new” cards in your entire collection. 
Thanks, that’s comforting.
I am already using that method, indeed not something we should have to do.
I have found an alternative for the latest expansion only: I don’t (mass) disenchant any latest expansion cards until the next expansion comes out. I still get the “new” card bug for Classic set due to the steady stream of Classic packs, as well as other expansions if they’re handing out free packs.
But not for the latest expansion anymore since I save them up, also to get more dust for nerfs, which are most likely to happen for the most recent expansion. Again avoiding disenchanting should not be the regular way to manage collections due to the constant ‘sparkling’ animation, but it is now what I’m forced to do by this bug.