At this time the server doesn't trigger notifications for mounted/shared folders. So a manually refresh is needed. We are looking at ways to allow notifications to be sent in a case like this (where the person making the change is notified). If we just notified all the shares it could be a scale problem since some calendars could be shared with 100's of people.
I filed this as an enhancement.
http://bugzilla.zimbra.com/show_bug.cgi?id=6082