ISC DHCP server config option stash-agent-options
The stash-agent-options statement
If the stash-agent-options parameter is true for a given client, the server will record the relay agent information options sent during the client's initial DHCPREQUEST message when the client was in the SELECTING state and behave as if those options are included in all subsequent DHCPREQUEST messages sent in the RENEW- ING state. This works around a problem with relay agent informa- tion options, which is that they usually not appear in DHCPREQUEST messages sent by the client in the RENEWING state, because such messages are unicast directly to the server and not sent through a relay agent.
It is a real issue (for DHCPv4, in DHCPv6 the server controls the use of unicast (i.e. direct communication) by the client) but I have no idea how (or whether) it is handled by Kea...