I recently reached 500 wins on Paladin and when I went to redeem the achievement in the achievements menu, I was granted a golden Tirion Fordring. I inspected the tiles for similar achievements for other classes and noticed that they advertise a reward of “Golden Legendary Core Cards [Class Name] Class.”
Before this was implemented, I was already past the 500 win milestone on Warlock and I do not have a golden Jaraxxus.
My assumption is that players already past the reward threshold are not being granted the updated rewards.
There’s a long list of known issues that, of course, is not posted in the Bug Report forum but somewhere else where you would never expect it.
You ran into two issues both mentioned in that post.
1: A correction to the number of wins from Duels and Arena, pushing you past the 500 win mark for Paladin; and
2: Receiving notification of getting golden versions of cards in the Core set (which you should as reward for achievement), but you cannot yet see those cards because the Core set is not live yet.
The golden cards you unlocked should become visible once the Core set is live.