How do I replace the post title with a custom field?

The acf/update_value filter won’t work as it’s meant for modifying the acf value and not the post it’s associated with. Try using the acf/save_post action instead. Here’s a simplified example of how I’d normally set the post title from first and last name fields in acf.

add_action( 'acf/save_post', 'biwp_set_title_from_first_last_name', 20 );

function biwp_set_title_from_first_last_name( $post_id ) {
    $post_type = get_post_type( $post_id );

    if ( 'contributor' == $post_type ) {
        $first_name = get_field( 'biwp_first_name', $post_id );
        $last_name = get_field( 'biwp_last_name', $post_id );

        $title = $first_name . ' ' . $last_name; 

        $data = array(
            'ID'         => $post_id,
            'post_title' => $title,
            'post_name'  => sanitize_title( $title ),
        );

        wp_update_post( $data );
    }
}

Note the priority of 20 used in the action. A priority of less than 10 will hook into the acf/save_post action before ACF has saved the $_POST data. While a priority greater than 10 will hook into the acf/save_post action after ACF has saved the $_POST data. I’m using 20 so I can access the acf data that’s been saved.

Looking at the docs, Version 5.6.0 of acf has just added a parameter called $values which is an array containing the field values. You could use this to get the acf values directly instead of using get_field.

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