The general idea was that Bitcoin client is supposed to randomize its outgoing IP address usage to avoid constantly connecting to the same subnets. The code is a little weird, it avoids connecting to the same /16 subnet over IPv4, what it effectively does with IPv6 I did not analyze. But even with regularly changing IPv4 addresses (like vast majority of xDSL deployments, every 24 hours) it doesn't operate well, certainly is much worse than Bittorrent at locating and connecting to peers.
I set 4 IPv6 addresses on the /64 subnet that I got from my providers. But I use only 1 IPv6 address for the node and I made sure that both incoming and outgoing Bitcoin packets use that particular IPv6 address via ip route and iptables settings.
According to what you mentioned, does this mean we should not set 2 different IP addresses on a single Bitcoin node even they are on different network, i.e. IPv4 and IPv6?