Custom Fields not showing up for custom post type

If you have created an Class in your plugin then you should use the action hook for admin_init in class only just like you have used “init”

function __construct() {
  add_action( 'init', array( $this, 'register_custom_post_type' ) );
  add_action( 'admin_init', array( $this, 'add_artbeat_meta_boxes' ) );
  add_action( 'save_post', array( $this, 'save_artbeat_custom_fields' ) );
}

Hope this will help you out.