Had an issue where changes to posts (and pages) were not appearing when viewing the preview before saving. It happened only on a page template that was had a lot of ACF fields and only on a live environment, while running exactly the same code as locally and a test environment where the preview function worked fine. After digging around forever, I read something about post revisions being related to the preview functionality. While still confusing as to why, it did indeed make a difference wether post revisions were enabled/disabled. After changing wp-config.php to enable revisions, you might still first have to save the post once to actually create a revision. Hope this helps someone!

Enabling revisions in wp-config.php:

define( 'WP_POST_REVISIONS', 0 );

define( 'WP_POST_REVISIONS', 1 );

This applies to WP 4.8.5 and ACF Pro 5.6.10.