I hope you can help me see what’s wrong here! I am finding that when I create new posts of a custom post type with a custom meta key, unless I actively click that particular meta option when creating the post, the post doesn’t appear in meta_query filters. If I just create a new post of that type but don’t click anything, so the value is left as the registered default, it’s ignored.

I have a custom post type initialised on the “init” hook like so:

add_action( 'init', array( $this, 'create_post_type_and_meta_fields' ) );

In that function, as well as creating the post type itself, I am registering meta fields like this:

$fields = array(
    array(
        'field_name' => 'start_date',
        'field_type' => 'string',
        'default' => '',
    ),
    array(
        'field_name' => 'location',
        'field_type' => 'string',
        'default' => 'in-person',
    ),
);

foreach ( $fields as $field ) {
    register_post_meta(
        $post_type = 'event',
        $meta_key = 'event_' . $field[ 'field_name' ],
        $args = array(
            'show_in_rest' => true,
            'single' => true,
            'type' => $field[ 'field_type' ],
            'default' => $field[ 'default' ],
            'auth_callback' => function() {
                return current_user_can( 'edit_events' );
            }
        )
    );
}

Then in the admin I am filtering for these meta values like this:

public function process_filters( $query ) {

    global $pagenow;
    $post_type = $_GET[ 'post_type' ] ?? 'post';

    if ( is_admin() && $post_type == 'event' && $pagenow == 'edit.php' ) {

        // Get the meta query so far
        $meta_query = $query->get( 'meta_query' );

        // If it's empty, start a new one
        if ( empty( $meta_query ) ) {
            $meta_query = array();
        }

        // If applicable, add a filter for location to our meta query
        if ( !empty( $_GET[ 'filter_location' ] ) ) {
            $meta_query[] = array(
                'relation' => 'OR',
                array(
                    'key' => 'event_location',
                    'value' => $_GET[ 'filter_location' ],
                    'compare' => '='
                ),
                array(
                    'key' => 'event_location',
                    'value' => 'hybrid',
                    'compare' => '='
                )
            );
        }

        // Save the updated meta query
        $query->set( 'meta_query', $meta_query );
                            
    }
            
    return $query;
    
}

These filters work perfectly when $_GET[ 'filter_location' ] matches an option I’ve selected for the post, but do not match anything when the post just has that value as the default.

However, outputting get_post_meta( $post_id, 'event_location', true ) for a post where I didn’t choose an option DOES return the default in-person, as it should.

Any ideas? I’m afraid it’s one of those!