leaf tip burn can happen anywhere roof top or yard planted especially on younger plants... and it has been a brutal summer for many of us ..the new leaves are your second flush ..first is is spring ( obviously)is "major" and second about now in many areas ...the second flush will usually stay small and with mine it is sparce and it being tender can get destroyed buy the sun if it's really hot and sunny... although i had a couple that put on alot of new top growth about three weeks ago..I don't think what you are experincing is unusual. Others may have additional or better imput ..David

I believe the new growth is a response to the increased watering combined with the shadier location, making the water go further. Acer palmatums are genetically set up to take advantage of the southeast Asia summer monsoons. The increased watering is duplicating that. I think it is a little too late in the season to add a lot of fertilizer.
The color of new growth is a selection factor when growing Japanese Maples from seed, not just fall color.