A Guardian of Hoshkar commonly fall beneath Hoshkar's platform and become unattackable. No doubt we have all dealt with this since day one by keeping them on the ground level. Nevertheless, it seems to be a pathing oversight.
They are fine if you fight them on the same terrain they spawn on. They fail to walk up the grey steps and always fall through as you stated. The only reasonable way to recover them once this happens is run to the lava where they will pop back up to normal Z-radius.