What database state changes happen after a post is manually "updated" with no changes?

Question

After importing a series of posts (using WP All Import), my (custom) page template is unable to access the post's contents/fields. However, if I open the post in the editor and click "Update" without making any changes, the page template suddenly works perfectly. This makes me assume that some previously non-essential and previously empty database field is inserted.

Although this "bug" is probably due to a nuance in either the Advanced Custom Fields plugin or WP All Import plugin, my testing still suggests that some very minor database state change is happening after clicking the update button (even without making any changes).

Edit: FYI after importing posts, I've tried deactivating all plugins and the issue still persists.

Edit #2: After more testing, it seems that WordPress generates _fields after manual "updates" but not during the import process. Example, importing data into field full_address WordPress will create a _full_address only after a manual update. I'm doing more testing now, but this might be the problem.

Answer 1

To answer my own question, WordPress does not appear to make any changes to the database after a post "Update" that contained no changes. Any changes that happen should be the result of a theme or plugin.

However, the Advanced Custom Fields (ACF) plugin does. Specifically, if ACF fields are set (such as through WP All Import) without also setting the ACF field key reverence, it appears that a click of the "Update" button will add these field key references.

When I attempted to add field key references to my import, I still had issues. Therefore, I found the easiest solution (instead of clicking "Update" on several hundred posts) was to run the following:

function bulk_update_posts() {

    $args = array(  'post_type'=>'post', // Change 'post' if you're using pages or a CPT
                    'posts_per_page'   => -1,
                    'post_status' => array('publish', 'future') // Includes published AND scheduled posts
    );

    $all_posts = get_posts($args);

    foreach($all_posts as $key => $one_post){
        $meta_values = get_post_meta( $one_post->ID);
        foreach($meta_values as $meta_key => $meta_value ){
            update_field($meta_key, $meta_value[0], $one_post->ID);
        }
    }
}
add_action( 'init', 'bulk_update_posts' );
//add_action( 'wp_loaded', 'mass_update_posts' );