Misaki sorry for the delay, i had to do some digging into the code as this game was developed by another team and we are maintaining it now. 
I don't think we are resetting the slots. We are just setting the animation when using it. Also, it seems the skins are being used on a weird way where the texture is replaced by another texture on the fly. (see example attached)
Here is the code to set the skin:
    public void SetTextures(Texture textDown, Texture textUp, bool head)
    {
        if (head)
        {
            headDown.GetComponent<MeshRenderer>().material.mainTexture = textDown;
            headUp.GetComponent<MeshRenderer>().material.mainTexture = textUp;
        }
        else
        {
            try
            {
                Debug.Log($"Setting up {textUp} / down {textDown}");
                bodyUp.GetComponent<MeshRenderer>().material.mainTexture = textUp;
                armUp.GetComponent<MeshRenderer>().material.mainTexture = textUp;
                bodyDown.GetComponent<MeshRenderer>().material.mainTexture = textDown;
                armDown.GetComponent<MeshRenderer>().material.mainTexture = textDown;
            }
            catch (Exception e)
            {
                Debug.LogException(e);
                throw;
            }
        }
    }
Here is the code to set the animation:
    public void SetCorrectAttackAnimation()
    {
        if (this._baseItem.fullTroopData.alignAnimations)
        {
            this._baseItem.SpineRenderer.disableAutoUpdateAnimation = true;
            this._baseItem.state = Common.State.ATTACK;
            this._baseItem.SpineRenderer.SetAnimation("attack", false, this._baseItem.fullItemData.defenseConfig.attackTimeScale);
            this._baseItem.LookAt(this._currentTarget);
        }
        else
        {
            this._baseItem.SetState(Common.State.ATTACK);
            this._baseItem.LookAt(this._currentTarget);
        }
    }
This is the base texture

This is a skin in the game that is loaded in the fly and replaces the base avatar texture
