Heavy stun on Xesht makes him immune

Xesht become untargatable right after he gets heavily stunned.
I assume he suppose to become untargatable after stun,go underground and then reappear, but right now its not.
Im playing phys build and only way to have decent window for damage is a stun, and its wasted all the time.
Kind wasted 10 breachstones already.
Not even that, the moment he becomes "heavily stunned" animations and few projectiles become invisible but still doing damage, like slap hand, hands from undergrounds, fireball