I've found that g-sync does not work properly, either. I feel like it worked a few days ago, but I'm patched now so that may be it.
Using latest Geforce drivers and clean install
Scenario 1:
NVCPL: G-sync on for fullscreen, v-sync forced ON in 3D settings, g-sync indicator enabled
POE2: Fullscreen
Result: FPS exceeds refresh rate (144) and G-sync not enabled
Scenario 2:
NVCPL: G-sync on for fullscreen and windowed mode v-sync forced ON in 3D settings, g-sync indicator enabled
POE2: Fullscreen, Windowed
Result: FPS syncs with refresh rate (144) and G-sync enabled
Either POE2 fullscreen is not exclusive, or something changed.