That’s interesting.
DC-Fix itself doesn’t cause many losses.
With such setup you get a lot of light inside the lens going astray. It surely makes the beam wider, reducing intensity. It may increase bezel losses. Reflection losses might be higher, even some TIR.
But at the same time…manufacturers do it. It shouldn’t be bad. Maybe the amount of diffusion should simple be smaller?

http://www.kunruioptics.com/images/KR100-1B1.jpg