Platform, device version and operating system
Steam (using Win7 if relevant)
Screenshot or image
not needed
What you were expecting to happen, and what actually happened
I was expecting that my hero (level >1200) will have 12 attack as before the 3.5 patch, now he has only 10.
Side note: Hero screen also shows “+10 Base Skill”
How often does this happen? When did it begin happening?
Happens always in the Arena, started after the 3.5 patch.
Steps to make it happen again
Play Arena
In 3.4, your guild bonus was applied. This is silly, because it gives players who are in a high-level guild advantages over players in a lower-level guild, and Arena is meant to be a place where such things don’t apply. So now, your guild bonus is NOT applied. I’ll bet your hero screen shows “+2 for Guild Bonus”, right? That’s why you used to have 12.
My guild bonus was (and is) +3, but prior the 3.5 patch I had 12 attack not 13. Are you sure that is what was changed? I haven’t seen any mention of this in the release notes.
i was told arena wasnt nerfed
Maybe it wasn’t targeted but got hit by collateral damage. 
Yeah this is wacky,and was not in the patch notes.