Hello Marcos
I don’t have a clear cut answer for you at the moment, however, doing some research, I’ve found the following Cisco documentation.
https://www.cisco.com/c/en/us/td/docs/routers/asr903/software/guide/ip/16-6-1/b-dhcp-xe-16-6-asr900/implementing_dhcp_for_ipv6.html#GUID-82004112-75D9-4114-A19C-B0B8B75DC21B
Now I know that you yourself have gone through documentation as well, however, if I may add the following comments, it may steer you in the right direction.
Among other useful information in this section, it states that:
IPv6 routes are added when the relay agent relays a RELAY-REPLY packet, and IPv6 routes are deleted when the prefix delegation lease time expires or the relay agent receives a release message. An IPv6 static route in the routing table of the relay agent can be updated when the prefix delegation lease time is extended.
Could it be that the KEA DHCP server is sending this RELAY-REPLY packet but it is not being perceived by the Cisco router? Is the DHCP server configured appropriately to have this packet sent in order for the relay agent to insert the appropriate routes to the destinations assigned this prefix? More generally, what I’m saying is, is there a possibility that the DHCP server being used may require some additinoal configuration in order for it to communicate correctly with the Cisco router. Can you attempt the same topology, but instead, use a Cisco router as your DHCP server and see if that allows the relay agent to install the appropriate routes?
I hope this brings you one step closer to the solution! Try it out and let us know…
I hope this has been helpful!
Laz