i was just wondering about that to! i am pretty sure it is normal it's happening to mine, but not that deep in. dont worry!
Took it back today and Apple replaced it no issues. They said it was not supposed to do that. Didn't take too long at all.
I am taking mine in tomorrow for that same issue. It does it all the way up the left side, but not the right. I will have to see what they say.