I have seen this problem on and off across different set ups in different locations, and across different versions of the Mac OS. The on consistency that I can see is Windows 2k8 server. The group memberships are empty in OD (whatever version of Mac OS and Server) with Windows 2k8. This was not the case with a Windows 2012 install I worked on recently. I am no Windows expert, so it may not be the Windows Server version but something in the configuration.
I hope this helps