Distorted Truth (Rupacitus) battle provides special secondary weapon called Spear Bomb, used to break bubbles and interrupt boss' swim phase. After today's update, hitting boss with the spear causes Abnormal gameplay detected error, preventing entire party from finishing the battle and gaining loot. That said, it effectively slows down each run and punishes everyone in team when someone doesn't know, forgets or does it on purpose. This also applies to Royal Army version of the battle, which includes plenty of inexperienced players.
Of course this is not intended behaviour and should be fixed ASAP.