Ya know, I also have one Acan frag, where the largest polyp also has a pinched area. It's been this way for a while now, but low and behold, i'm still receiving new babies, and it takes food well.
I recall investigating and wasn't able to come to a solid conclusion, but know that no matter where I put it, it just stayed this way. From what I see, the acan didn't come to me in this condition. Odd, but just wanted to reach out and give personal experience and send wishes that ours are still in good health!