UCS Domain Controller - Strange Domain name in Windows

I successfully installed pfsense, UCS and Windows Server 2016 on 3 different Virtual Machines.
UCS acts as the domain controller.
For the naming I used a subdomain of our public domain: “ad.xxx.de” . As Netbios: XXX. I could join the Windows Server to the domain and it works great. But there´s one strange issue:
Everytime somebody is logged in without Admin rights and higher rights are necessary the usual popup appears, pre filled with the domain name ready for the Admin to log in. But the domain there is called just “AD”. Why does windows not pick up the right name (XXX or ad.xxx.de) ? It seems like windows just uses the first part of the domain ad…
Thank you

Yes it shows the NetBIOS domain name.