Lagerstroemia indica rot

It doesn't matter at all. The state of the ball looks good, not rotten, like insect bites or diseases, which is not clear in the picture. If you don't continue to expand, don't worry. As you grow up, the wound will go under the ball. If it's a spider bug, get some flower protection medicine and spray it. If it is rust, use triadimefon. Or it may just be physical damage. Little problem. It won't kill you.