Private appointments are currently not sent to e.g. Outlook at all when they reside in a shared store.
These objects are removed from the object stream.
When they are marked as public later, ICS will export a modify flag which is passed to the client.
As the object is not known there (ignored before) it's not displayed/updated on the client.
Workaround: the shared calendar could be closed and opened again. After the resync all previously private (and now public) items should be there.
Reported at: 2017022210002886