macOS Server calendar delegation not finding users

This problem started at a client of mine after they upgraded clients from 10.6 to 10.7 and up. Calendar was stable before this. Webcal finds all ldap users, calendar app on clients does not find everyone, and I have yet to find a pattern when who doesn't find someone else. Some cannot find very old ldap users, others cannot find an newly created ldap user. This when creating calendar events and inviting other users. By the looks of it, a bug in the calendar.app in macOS.