Yes have that effect too ( from time to time ), it seems to be there when the flashbang does not explode in the line of sight but more beside the view-angle.
In my opinion that is absolutely O.K.
As long as the player doesn´t look directly to the explosion angle his vision should not be as affected.
That way players can react on flashbangs by turning away for a shot moment.
I wrote about this before... having been flashed a few times myself, the thing that's so debilitating about the flash isn't so much the flash as the sound. Simply turning away or not looking at the flash shouldn't render the victim unaffected. The piercing sound juggles the fluid of your bony labyrinth so much that it basically induces vertigo. You just loose your balance and orientation in space, plus your eardrums get molested (and also you get blinded if you happen to look). Its pretty intense. I mean some people piss themselves.
The best rendition of an actual flash mechanic in a video game I've seen was actually Ela's Grzmot mine in siege before they nerfed and changed it. Inducing that kind of extreme wobble and inversion of controls felt much more genuine than the typical white screen that usually finds its way into games.
I think the flash needs a complete rework. First off, it should induce a simulation of extreme disorientation, it should wobble the fov, distort the image, mess with player controls for a short while, regardless of whether you looked at the flash or not. Second, the flash of the grenade starts before the actual bang. A half second after you cook the grenade, it starts sparking just as brightly as lighting magnesium. It's quite blinding before it even explodes. The level of screen white-out in addition to the disorientation effect should scale with how long the flash was in the affected player's view. Perhaps a smaller oval if they just saw the sparks for a split second and turned away, but the whole white screen if they got a face full of the bang.
I think this would, in essence be a buff to the flash, but I think flashes need a buff. They're so easy for players to ignore in so many games, even though flash bangs have been designed, and are actually, extremely effective at their purpose.