This is an known issue and is due to the way that WP saves values when you click the “preview” link. WP does not submit custom fields when you do this. While the frustration it causes is understandable, There really isn’t any way that has been found to alter this behavior in WP.
There is one work-around posted here that I have not tried https://support.advancedcustomfields.com/forums/topic/preview-solution/
This same problem is reported for all custom field plugins, here’s an example https://wp-types.com/forums/topic/custom-fields-not-displaying-in-post-preview-mode/
