Craft Commerce auto SKU with product ID
I'm going to guess that the id for the new product entry doesn't yet exist when the sku is being generated....it's probably created at the end of the save process once Craft is sure the save will complete.

Do you really need to use the ID in the sku?  

(I don't immediately see an obvious in built way around it either, but perhaps someone cleverer than me will come along with a good idea).