Yah GG, you got flashed good. Nothing you can do there, it's a max distance throw going off at the last possible moment. It's literally the perfect flash.
I do not have many instances where a perfect flash happens and I'm blinded for a full 7s. But we're comparing our own experiences which might be different for a number of reasons.
Also did some testing, so the time for a flash to go off after leaving the hand is approx 1.5 seconds. It's closer to 3 seconds for the full pull out the nade animation, throw, and then explode. A grenade thrown through the air like that goes about 30m.
The grenade will not flash you if you're looking more than 90 degrees away from the nade. This is more than just "off the screen" since player fov is not 180 degrees. It doesn't matter if the grenade is at your feet, if you turn in time, you'll be unaffected.
The full 7s duration only happens within 2m at most and only if you're looking basically dead center at the flash. Looking off to the side or up but not a full 90 degrees reduces duration by at most 3 seconds, depending on how close to 90 degrees you are.
At 6m, the max blind duration is 5s. At 9m, 4s. At 12m, 3s. At 15m, 2s. These are all worst case scenario of the grenade being dead center of the screen.
Idk seems fine and will be even more fine once grenade indicator comes (assuming it isn't just for frags).