I added this function below. Although as I’m looking through the code again it looks like it might be trying to call the input_description function in my theme’s RWMB_Field’s abstract class instead of from the plugin’s.
/**
* Display field description.
*
* @param array $field Field parameters.
* @return string
*/
protected static function input_description( $field ) {
$id = $field['id'] ? ' id="' . esc_attr( $field['id'] ) . '-description"' : '';
return $field['desc'] ? "<p{$id} class='description'>{$field['desc']}</p>" : '';
}
Yup looks like wp-content/plugins/meta-box/inc/fields/heading.php is just missing the function to return the description. Adding the method fixes everything.