Frequently :-)) Virtually all Japanese maples are grafted and it is not at all uncommon for the rootstock to produce shoots and foliage as well as the top graft. And the two types of growth will be very different in appearance. Typically this is something seen mostly on young trees, however I just saw very mature tree displaying this feature. It was a green laceleaf that appeared to be planted right on top of another JM but one with a distinctly upright habit and non-dissected foliage. This was actually the root stock (generic Acer palmatum) onto which the green laceleaf had been grafted. The root stock shoots had been left undisturbed and had been allowed to grow up into a full sized tree that has now overwhelmed and permanently altered the growth of the laceleaf.

If you post a photo, we could confirm but look for any shoots developing below the graft union and remove ASAP. When still young they will rub off easily (rubbing off manually is better than using pruners).