I engaged a 1v1 fight with a LUX as Vayne. She threw her Q out and then used her ultimate. I was not hit by her q and was able to flash her R but when I flashed behind her I still took the damage from the ultimate ability. I definitely flashed before the Ultimate popped and again, landed behind lux and was still hit by the ulti.
You might have a bit of delay maybe, or possibly had a bad latency? This could also happen when you encounter "FPS Lag". It usually happens to me when I use my "Toaster Laptop" to play. Some actions are delayed. You could as well try recording this using http://na.op.gg just to make sure you flashed before the ultimate. That's in case you ever encounter this again. I hope I helped you.
Lux's ultimate seems like an forward-to-back laser, but it's actually center-to-sides, meaning it can give the illusion of escape when in actuality you're about to suffer a very colorful death. Since it's a from center-to-sides laser, the damage is dealth instantly after the red line goes away, even in the laser hasn't actually fired yet due to the animation of the actual laser. Lux's ult is also a very quick cast after the initial sniper laser, and I've had the misfortune of flashing a millisecond too late and getting blasted in the face even though it looked like I should have made it out unscathed.
