Brake installed correctly can impact harmonics, as BP said. Brake that has loosened up can definitely effect things.
Best thing to do is work up a new load with the brake on it and go from there. If your brake was loose, I'd take it off and clean the whole brake and your gun threads really well and then re-install and tighten down good. You may have to work up a new load or may not. My rifle shot the same loads good with and without the brake, but the POI moved a good bit (almost a mil if I remember right).