all my Ubuntu clients joined by the join assistant seems to use the DC Master as DNS server and not the site DC distributed by DHCP (systemd-resolve --status). Is the DC Master somewhere hard-coded as DNS server?
In a non-UCS network the Ubuntu seems to use the correct DNS server. But the Windows clients use the site DC as DNS, so the DHCP server should be working correctly.
All Ubuntu clients have still the host type “UCC” in the LDAP.
EDIT: There are some messages in the log of the local DC:
DHCPINFORM from $windowsip via br0: not authoritative for subnet $local-subnet