1.0.12 notes say that arena difficulty increases quicker.
I have been only once in the arena before, but now that I tried it again i reached level 50 and stopped because I wanted to take a break.
Also I had to go through a lot artifact loot screens at the end
Looking at the exp I was getting they seemed to increase so I though the enemies were getting tougher, but then I noticed that there were times I was getting low exp as well.
Not that they had much of a chance to attack me, only one enemy creature got to attack most of the time, thus I could progress so far.
But it didnt seem that they get much stronger, if at all