this is usually caused by not having physbones from the outfit set correctly. make sure the breast bones that are / were part of the outfit are in the "ignore transforms" section of the physbones component, or don't have their own physbone components.
if you have a base selestia model, just add one thing at a time till it breaks, then try to fix that part
i would suggest using Modular Avatar so you'll never have to worry about permanently breaking anything.