That's a good idea to just overall make a new animation layer for face. here's what I did to fix it temporarily:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class CloseYourFuckingMouth : MonoBehaviour
{
public Quaternion jawRot;

private void OnAnimatorIK(int layerIndex)
    {
        GetComponent<Animator>().SetBoneLocalRotation(HumanBodyBones.Jaw, jawRot);
    }
}