I searched bugzilla and couldn't find anything on this.. It looks like the admin console is assigning the value of zimbraDomainDefaultCOSId as the name of the COS rather than the ID. When I add a new user, the user simply gets assigned to the default COS. Setting the property with the ID via zmprov seems to fix the issue.
Server that I'm seeing this on is 5.0.15 RH5 x64. Can anyone else confirm this?