It would have been more helpful if you had let us know what kind of JM (the cultivar name) you are growing but it really doesn't make a huge difference ;-) Many different kinds of Japanse maples can put out some late season growth that looks very different from the rest of the plant........often just in coloration but sometimes also leaf shape and rate of twig growth. It is nothing to worry about.

I would assume your tree to be one that typically leafs out in spring with red foliage. The new growth is just echoing that spring growth. NBD