Does this work if you create a credential for your admin account and use the -Credential parameter on the Get-ADPrincipalGroupMembership cmdlet?
What about the account delegation setting on your admin account? Is (Get-Aduser <YourAdminAcct> -property accountnotdelegated).accountnotdelegated
true or false?
Over the past several years this problem has been reported and mentioned on quite a few places on various InterNet forums.
One of the suggested workarounds, if you find the creation of the credential inconvenient, is to just get the users' direct membership from the users' "memberof" property.