The usual cause of this is that the belt is not properly centered in one pulley and on that one pulley is off by one groove. I know you have checked that, but you may want ot look again. Especially look at whatever pulley is hardest to see. It is also possible that it was initially slightly off and moved into the correct position slightly nearly immediately after starting the engine. It is also possible that you have either a defective belt or a belt that was damaged somehow before it was installed. This sort of thing is not always obvious.
No matter what caused it, keep an eye on the belt and replace it if there is any doubt.
