I think the reason you're seeing flickering is that most loading screens drop in framerate below 60 FPS (occasionally, they can hit single digits), at which point the monitor will start to flicker due to a lower refresh rate being used with Gsync.
i got the flickering of the backlight on game session too. especially on Guildwars 2 or LoL and i probably stated that it happen witouth gsync too. and well sometimes gsync vanish from the control pannel.
I think flickering during loading screens is normal. Maybe at very low frames (30 and <30) they strobe the backlight in order to avoid pixel persistence. I wouldn't be too worried about it. Some other users are experiencing flickering while playing games with gsync. Now that is a problem. Thankfully, I havent seen that yet.
This is due to Nvidia G-Sync drivers or it's specific to the game code. Please either roll back to the last version or wait for the new version. There is no firmware update that can be applied to the PG.
PWM strobe only activates when ULMB is enabled; the frequency is reduced to match the display refresh, which is why it goes darker.