You have installed ShortPoint for Modern experience only and you do not see your Theme Builder changes applied in Classic Page preview mode.
ShortPoint Only installed for Modern Experience:
You do not see your Theme Builder changes applied in preview:
Since ShortPoint is not installed for classic experience when you navigate to a classic page while Modern Theme Builder is activated, we loose connection between the theme builder and the preview page (Since its classic and ShortPoint is not installed there).
In Theme Builder Preview Page (Right side), please make sure you remain only on Modern Page and when you navigate, please make sure the page you are navigating to is Modern page.