I researched and i believe, I know the reason.
The group is received by pfsense only, if:
-the created user in AD (e.g. 'vpnuser') is member of at least two groups (e.g. 'Domain-User' and 'vpngroup')
-if the AD/pfsense group (e.g. 'vpngroup') is not the default group of 'vpnuser'
Just tried to replicate this .. Strange, but it is as described above.
btw: the extended query still not working, but that is another topic :)
Edit: everything perfect right now, even with multiple extended queries.

