Just want to add that when you go to create a new deck and it asks you whether you want to import the one in the clipboard, if you choose "No" the game will clear the clipboard. So if you change your mind you have to get that string back into clipboard again and reenter the deck creation screen.
That's my experience at least. Using a PC, no idea about mobile.
it's not a hs client issue, it's the decklist that isn't getting copied to the clipboard when pressing the button on hearthpwn.
My analysis is the same. I'm on iOS and tried the button on hearthpwn, it said "copied to clipboard" but when I did paste on a random text window it was an old copied text that I did before. So it probably works in hearthstone app but it doesn't get copied from here.
I actually managed it to work now. You need to press the button "view deck string" and copy the text manually from the pop-up window. When you've done that and enter the app and starts a new deck you'll se the note if you want to paste your copied deck. I've done this with iOS and safari.