I tried deactivating the handful of non-GP plugins (on my local server, I did not try this on the live site – but I can try that too if you think it might act differently online), and still experienced the same thing:
1. Make change on a page
2. Click “preview page”
3. The “preview page” page refreshes without the changes
4. Click “preview page” a 2nd time
5. The “preview page” page refreshes and shows the changes
It happens when I’m editing the pages themselves (not in the customizer).