Gsync compatible does absolutely not gaurantee flicker free experience- all LG OLEDs are gsync compatible and yet all of them have near black flickering
What you just said stands against what NVIDIA themselves said many times over about their cert process - every certificate of the G-Sync family (G-Sync Compatible included) has a lot of requirements and that includes lack of any flicker related to vrr. They claimed it's one of the core tests they conduct, and any sign of flickering means they refuse G-Sync cert for that panel. However! That does not mean they can test every single scenario. And so far, every single VA or IPS or TN monitor I've seen with their cert had no shred of any flickering at all (unlike lots of Samsung FreeSync ones, which tend to flicker madly).
However, I have 0 experience with OLED for gaming (including LG OLED). Still, just a quick look at Reddit and I can see a few complaints about LG OLED, but also a lot of people who do not experience these issues. Some claim newer TV firmware fixed it for them fully. Without seeing and testing it myself, I can't say what might be causing this, but I definitely will do it when I get one in my hands.