Displays equipped with Nvidia's G-Sync variable-refresh tech are pretty great. But they're not perfect: some users have been reporting slight flickering in some games.
The guys at PC Perspective did a little sleuthing this week, and they've both confirmed the problem and identified its cause. Turns out the issue has to do with the way G-Sync handles "stalls" in game animation—that is, cases where the frame rate briefly dips to zero, as on some loading screens or when content loads in the background. G-Sync displays can't simply stop refreshing the image when that happens, so a failsafe measure kicks in:
Completely stopping the panel refresh would result in all TN pixels bleeding towards white, so G-Sync has a built-in failsafe to prevent this by forcing a redraw every ~33 msec. What you are seeing are the pixels intermittently bleeding towards white and periodically being pulled back down to the appropriate brightness by a scan.
PC Perspective measured this effect by taking continuous brightness readings in EVE Online with an Asus ROG Swift monitor. (In that game, the problem rears its head when the user snaps a screenshot.) What happens is basically a "very slight brightness variation," which the site neatly graphed over time.
So, is there a fix? Not really. PC Perspective says all of the variable-refresh displays it's tested exhibit the same problem to some degree, and Nvidia is chalking up the problem to the way LCD monitors work. "All LCD pixel values relax after refreshing," Nvidia told the site. "As a result, the brightness value that is set during the LCD's scanline update slowly relaxes until the next refresh."
If better LCD panels aren't the solution, then perhaps Nvidia simply needs to cook up a better algorithm for handling frame-rate stalls.