Alright, update.
It's definately the way KiX looks it up.
If you try it using LDAP it works fine. for instance:
 Code:
Dim $objSysInfo, $objUser, $objGroup
$objSysInfo = CreateObject("ADSystemInfo")
$objUser = GetObject("LDAP://"+$objSysInfo.userName)
$objGroup = GetObject("LDAP://CN=Domain Admins,CN=users,DC=mydomain,DC=local")
If $objGroup.IsMember($objUser.AdsPath)
  ? "Global Test Succeeded"
Else
  ? "Global Test Failed"
EndIf