Same here. Both solutions above worked.
1/ Exit Zwift
Navigate to \This PC\Documents\Zwift
Find the file “prefs.xml”
Right-click and “Edit”
Change
[FULLSCREEN]1[/FULLSCREEN]
to
[FULLSCREEN]0[/FULLSCREEN]
(it’ll be pointy brackets, not square brackets, but I can’t use those in this forum)
save, exit, restart Zwift
But that solution left me with a windowed Zwift and didn’t fix the root cause
2/ Right-click on the desktop
Choose “Display settings”
Choose “Advanced display settings”
Set the refresh rate to 60Hz
or
3/ Right click on the desktop
Choose “NVIDIA Control Panel”
Choose “Display → Change Resolution”
Set “Refresh rate:” to 60Hz
Then put Zwift back to fullscreen, either by reversing approach 1/ above, or from the settings in-game