Router can use ASIC to hardware loop up the destination port for a receiving packet, if it is the port the packet comes in, router will consider to send ICMP redirect packet, however, due to sending to the SA of the packet doesn’t make sense, so router will check if the SA is in the same subnet with the router. I believe our router can’t do this in hardware, that’s why we see high cpu. And in your test 1, CPU found the SA is not in the same subnet of it’s interface, so didn’t send the ICMP redirect. In test 2, SA in same subnet, so router send ICMP redirect.