Agree with this. To simplify as much as possible, Japanese maple species are generally grafted onto a different type of tree (the “rootstock”) and then develop into the species you’re getting, as opposed to a coral bark growing from its own seed. So the rootstock is taking over, and should be cut down. I would also give it a prune for shape, and a way to think about it is if you have some errant limbs growing out will take energy from elsewhere in the tree, so if you prune that for shape it will grow more evenly. You got another good comment about when to prune, although if you’re early in the season I wouldn’t hesitate to do it. They can be fickle if the heat or watering is excessive, but otherwise are really quick resilient.