I am seeing the same thing.... but I just concluded it was as expected because what I am seeing is the /stopcasting stops the previous spell but the GCD is still in effect. So if I hit my heal macro with stopcasting at the front, if it is within one second of when my previous spell was cast it will just give me an interupted on my cast bar, then I hit again and it casts the heal. But if it has been more then 1 second since the first spell it will go straigt to casting the heal.