Generic problem reported upstream: https://secure.phabricator.com/T7661
When a task is edited, the (new/"new") status of the "Security" field is notified regardless of it having been changed:
scfc triaged this task as "Normal" priority. scfc claimed this task. scfc set Security to none.
The first few times I feared I might have accidentally changed the setting.
Changes of that setting should only be notified if it has indeed been changed from the previous value.