Multiple Sessions - User Group Membership not applied

Hi Everybody

We're using Remote Desktop Services on a Windows 2008 R2 Enterprise Server.
Multiple Sessions are allowed, so the user xy can log on multiple times.

Now if we add user xy to a new group, the membership does not apply - even if user xy is logging out and in again. It’s also not working if you wait until the kerberos ticket lifetime is reached.

Only if all logged on users with account xy log out and log in again, the group membership is applying.

I’m not sure, if this is a problem/bug or if it’s related to the kerberos ticket store mechanism...
Does anybody of you have an idea why this happens?

Thanks for your help
Regards, Florian