My first thought is that twice weekly watering is too much unless you live in a very dry climate. Too much water will encourage leaf blight and root rot. Feel the soil and do not water unless it is dry to the touch when you poke a finger down to the second knuckle. The other common cause is sunburn or heat stress. Even in dappled shade a bit of sun blast can damage foliage on maples not used to the heat. You can remove the damaged leaves and I am optimistic the tree will survive. Keep growing, Marianne Binetti