We were also having this issue on ACF Pro 5.5.12, but fixed it by turning back on post revisions in wp-config.php. The code in wp-config.php is now define( 'WP_POST_REVISIONS', 3 );. We use WP Engine who turned this off by default which is why I’d guess it wasn’t working.
Can someone from the ACF team give us an idea of why this wasn’t working without having revisions turned on? It sounds like the way you handle preview is through the post revisions, but we’d love to know if this is always going to be the case since we plan to use WP Engine for hosting the vast majority of our clients.
Thanks a ton. Your plugin is unbelievably awesome.
Jonathan
