My friend and I were progressing through the Vesper's Host dungeon, at the part where you have to deposit the 3 radioactive orbs into the chests in the center. We were heading for the third orb when every enemy in that area became "immortal" — immune to all damage. We tried to clear every enemy in the center (but they spawn endlessly), there was no active radioactive orb (we had already deposited two), and this was supposed to be the location of the third orb. The door to pick up the orb doesn't open until the two large lemon-yellow enemies are defeated. Did we do something wrong? (On the way to the second orb, a small robot with only a sliver of health left also became immune to every weapon type.)