I found the missing piece of configuration; I was missing a source nat rule.
The config-wizard had created a source based nat rule like below:
rule nsw-src-interface {
match {
source-address 0.0.0.0/0;
destination-address 0.0.0.0/0;
}
then {
source-nat {
interface;
}
}
}
I just copied it and changed it to IPv6 like this:
rule src_ipv6 {
match {
source-address 0::/0;
destination-address 0::/0;
}
then {
source-nat {
interface;
}
}
}
Success!