Ability to always-respond to all requests in HA active-active mode
My impression is that ISC KEA doesn't always respond to all requests. I think this is due to the 1/n split.
I run two KEA instances sharing a single BGP anycast /32 IP prefix. DHCP Requests get routed via a DHCP relay towards the closest ISC KEA instance according to BGP. Load balancing is externally handled. This means KEA should respond to all requests it receives and not impose any load-balancing logic.
I think this is where the magic happens 
From my understanding active_servers needs to reflect the current server instance id (pri,sec).