In previous versions, the CardDAV interface sent simulated contact groups vCards to the client. This was done to achieve some kind of contact grouping for the Mac OS Addressbook client for contacts in the default contact folder and the global addressbook.
With an upcoming release, these simulated contact groups are no longer sent to the client, resulting in those groups no longer being displayed in the Mac Addressbook client.

The previous simulated folder groups are reported as 'deleted' to old clients requesting the associated resources on the server, leading to the groups being removed from the clients afterwards.