It's not just Line in the Sand that's having that issue. If you use some sidearms or fusion rifles at the same ranges in the Tribute Hall, you can get the same effects with them too. There's some sort of late registration that heals the enemy after a second, but if you deal rapid damage and deplete the HP to 0, the enemy will still die, albeit in a laggy fashion.
Yes, I agree also the sight seems off somehow not sure how to explain. I do like the gun but it is a bit -blam!- sometimes.