I'll add my voice to this.
(In my case, an official-G-Sync-module monitor (LG 32GK850G) running on an nVidia 1080ti)
Specifically, it seems the game will very easily switch into a "Borderless fullscreen" state, by alt-tabbing, but also sometimes just by happening to click the mouse while the game is loading, causing focus to shift before the game initializes the viewport.
I've made a habit of going into the graphics settings, switching to Borderless mode, applying, accepting, switching back to Fullscreen mode, applying, accepting, every time I alt-tab or launch the game, to try to force G-Sync to work.
Even then, I get the feeling it doesn't always "take".
Is it really this difficult to ensure that a true fullscreen viewport mode is enforced when the option is selected? I know this is a recurring and thankless issue with the likes of Unity Engine, but I would've thought this would be one clear benefit of having your own in-house developed game engine.
(Edit: I ask as a genuine question to developers - it seems like a LOT of modern game engines struggle near-endlessly with this issue, since some point in the past where true-fullscreen stopped being the default, and everyone switched to borderless-window as the default. So I wonder if it's some significant technical hurdle forced on developers by the Desktop Window Manager in later editions of Windows, or something along those lines?)