Keep an eye on your event rewards . . . After an hour of the game not loading, I was able to get in a few minutes ago. I hit an arena win milestone and received no event rewards for it.
Update: I just checked the achievement tab and it shows one less win than it should, so maybe the rewards will come with my next win. The win #s had been in synch at reset, so one of my recent wins must not have counted toward the achievement.
Update: Yep, rewards came with the next win. So just a small glitch of a single win not counting toward the achievement.