This happens because we are checking the source IP of the redirect message. If it is not the current first-hop gateway for the specified destination, then it will be ignored. For example if there is no IPv6 default gateway configured, the ProxySG appliance will ignore the redirect and mark it as bad.
In the redirect example above the source of the ICMPv6 was the default gateway (fe80::6), and the target of the redirect was fe80::8. Hence, it will add the route.
Here is an example configured default gateway: