Chin nerd here, I could be wrong but I believe you're referring to the mentalis INSERTION around the gingiva vestibule. The mentalis origin is connected to the menton (chin bone region). It's amazing I have issues with my mentalis muscle in that it makes my center lower lip too BIG, like it's creating an inverted V, yet if anything, only my mentalis origin could have been transected in surgery (submental chin implant approach).