UPDATE: I used a Power Pack right before level up and then used a Kamikaze. All experience carried over so this issue isn't affecting all accounts. It could be another experiment where only a certain percentage of players are affected or it could be a bug on only some of the servers.
One of the more useful things about the Arena is that you don't level until the fight is over. This means that stamina which would normally be wasted on a level up can be used and the experienced earned carried into the next level. The Arena is pretty much the Experience Bandit of stamina
[1]. Some players are reporting horrific ratios in the Arena because the excess experience over a level up is not applying. I'm not online and don't have time to test it out. Please comment with your findings .. Hopefully this is just a bug or a ridiculious experiment and things will go back to normal soon. Thanks to those who commented of experienceing this on our fan page.