The damage wasn’t orange, so that means you hit a body part that isn’t covered in Hellfire.
However, hitting the Body would also contribute to the breaking of the Back part, so it’s likely the case that your GS swing hit the Body instead of the Back and thus broke the Back while at the same time not getting the extra damage from hitting a Hellfire part since you technically didn’t hit a Hellfire part. The closer you are to the monster while wielding a melee weapon that has a long reach, the more likely your weapon can clip through the “surface” and hit something else on the inside or the other side of the surface.
The hitbox of this game is not the best, given how irregular gunlance wyrmstake hits are.