Maybe look at it in Wireframe mode (without the original model) to see if there are any inside Faces that shouldn’t be there. If you can’t see anything that’s wrong, then maybe there are double Vertices where those edges appear. Be careful when using M > Merge by Distance. I would not select the whole Mesh when trying that, because that might mess up the mouth area, where Vertices are already very close to each other.
Could be something else though…If you post a link to the .blend, we can have a look.
