Comment 1 for bug 1719806

This seems not a bug(, or this should be an RFE for automatically allocating addresses in IPv4 subnets?). Things go a bit differently with IPv6 auto addressing.

If a newly created subnet supports auto-addressing(IPv6 SLAAC, DHCPv6 stateless), all the internal ports on the network would be automatically updated with an IP address from that subnet. However this is not applicable for IPv4 subnets and IPv6 stateful. Thus existing ports won't be updated when you create a new IPv4 subnet.

If you try in the order that IPv4 subnets are created first then IPv6 stateful subnets, existing ports won't be updated with addresses from the newly created IPv6 stateful subnet either.

For newly spawned VM, if a port is created in a network which contains multiple subnets without specifying fixed_ips(i.e., not specifying subnets), it would be created with multiple IP addresses, one allocated from each subnet.