Here is a copy of what I sent to support:
In going past rank 399 in the Arena, I did not get my prizes. The screen froze on the description of the rewards. I never got the Dragon reward nor the common card pack. I tried emptying the app cache, but it did not fix it. I was in Facebook Gameroom when this happened.
Update: I tried losing some battles to drop my Rank back into the 200-399 Rank, in hopes of regaining my prize that way, but even at Rank 386, it kept me in the 400 to 699 Rank fighting the Rank#396 player.
I finally lost enough battles to rank back in 200-399.
However ranking back into the 400-699 did not give me my missing lost rewards.