Homie did you use "flat chest" Rather than "forever-flat" cause if you did and while lactating yeah, make sure your male character doesn't lactating to keep that "flat chest" Or edit your male breast to "forever flat" Or turn off the breast expansion perk if you have one active "milking skill" Okay.
Question: Do you have a mod active? If there is, turn off male lactating mod if you have one.