That's probably an abnormal data check, since if you could lived long enough till the end of the Raid Instance, it would check whether the expected damage dealt to the boss is correct or not. By turning on the dmg multiplier, it's the same as you're messing with the dmg output even if it didn't increase your total dmg at all.
It also have the same mechanism as for other instances, if you dealt way too high of dmg, it won't count the results and deemed it as failed attempts.