Fix so you can preview ACF field changes in Wordpress admin

functions.php

<?php
/*
   Debug preview with custom fields
   Taken from: http://support.advancedcustomfields.com/forums/topic/preview-solution/
   See also: http://support.advancedcustomfields.com/forums/topic/2nd-every-other-post-preview-throws-notice/
*/
add_filter('_wp_post_revision_fields', 'add_field_debug_preview');
function add_field_debug_preview($fields){
   $fields["debug_preview"] = "debug_preview";
   return $fields;
}
add_action( 'edit_form_after_title', 'add_input_debug_preview' );
function add_input_debug_preview() {
   echo '<input type="hidden" name="debug_preview" value="debug_preview">';
}

I searched for this answer for a while!
I finally found it. No code in the function required :

Enabling Revisions for the Custom post types solved this issue!

Ref : https://support.advancedcustomfields.com/forums/topic/preview-changes-not-working-for-acf-cpt/

Here, you find the latest solution to this bug by CamiloAPR. Worked for me but this is still an open ticket so one should always look into this ongoing forum thread ;)

Awsome

I searched for this answer for a while! I finally found it. No code in the function required :

Enabling Revisions for the Custom post types solved this issue!

Ref : https://support.advancedcustomfields.com/forums/topic/preview-changes-not-working-for-acf-cpt/

Thanks! Not having revisions enabled was my issue! I use WP Engine and they disable revisions by default (you have to ask their tech support to enable it). Then you need to enable revisions for each of your post types as well.

Thank you. In 2023 it still works.

This appears to no longer work as of June 2023. WP 6.2.2, ACF Pro 6.1.6. Revisions enabled.

We noticed today that WordPress's "Preview Changes" feature wasn't working correctly for ACF fields; the preview was showing their published values rather than the previewed values.

We tried out several suggested PHP snippets but found that they didn't resolve the issue.

After seeing this suggestion:

define( 'WP_POST_REVISIONS', 3 );

We did some testing and confirmed that the issue occurs when post revisions are disabled. In our case, it was due to our WordPress host (WPEngine) disabling post revisions by default. We were able to fix it by contacting their support and having them set the number of post revisions to 3.