Seems there were some recent changes in the WebUI, as a result the Business Entity annotation cannot be manually updated by a single click on the landing page of the Internal Volume. Whether this is an intentional change or a bug is not clear to me, I intend to investigate.
What seems to work, is to find the internal volume in question in a metric query, select (tick the checkbox) in the query results for the internal volume and assign the new annotation value for Business Entity by using the "Bulk Actions" => "Add Annotation" button in the upper right corner of the query results.