Seartooth Vyrantus turning invisible

I'm playing on PS5, fighting Seartooth Vyrantus. When I hit a combo with "Dark Squall" on a weak spot, the dragon vanished. The set up was "Heavy Draw" by Harding. I could still fight and target him, but he could not be seen anymore. I reloaded and had the same thing happen again. Had to beat him without using the combo.