You probably opened that pack a long time ago already.
Arena rewards are not added when you view the animation, That animation is just you seeing the rewards you already received.
The actual act of adding rewards to an account is done in the same second that a run ends. Often that is the end of a game (3rd loss or 12th win), but sometimes it is a force-retire.
In your case, you probably had an active run you forgot about. One day (months or even years ago) that run was force-retired. When you logged on, you found a pack, and perhaps some extra gold or dust. You didn’t notice the gold or dust and you might or might not have noticed the pack - but if you did you probably attributed to something else and didn’t think about arena.
Now you finally went to reveal the rewards. You were shown a pack and none was added. That’s because you already received it a long time ago.
EDIT to add this link: Hearthstone Common Issues