The "Services: Kea DHCP: Leases DHCPv4" shows to which interface the IP-address is assigned.
The "Services: Kea DHCP: Leases DHCPv6" shows only "LAN" as interface.
Have I done something wrong?
Apparently there are no IPv6 leases on any other interface?
Make sure that your device supports DHCPv6. For example, Google devices currently only support slacc, which will not appear on KEA IPv6 leases.
Many IoT devices only support SLAAC, if they support IPv6 at all.
Other than that, you have to select the correct RA mode to instruct devices to use DHCPv6 for all interfaces where you want it.
To me, it does not make much sense to use DHCPv6, even if you want to identify devices, because with IPv6 privacy extensions and randomized MACs these days, you cannot effectively do that anyway. Therefore, I prefer to use SLAAC only: https://forum.opnsense.org/index.php?topic=45822.0