It's natural for the formula/milk to drop a bit getting towards 11/12 months and as solids increase but the idea of milk (formula now, cows later) being accepted in a cup is more to do with when the bottles are gone (12 months if you want to follow guidance although you know many people keep a bottle long after this) and LO still needs a milk drink. Even after 12 months they should be drinking two servings of milk per day, that's why I said it's useful to get used to having milk in a sippy cup. It's not vital to offer milk in a sippy now, just handy, and nothing 'wrong' with it.
i didn't give DS milk in a sippy until he was 12 months when I weaned the BT bottle.