From reading the odhcp6c client docs and code I understand that it can preserve a state, however when my Internet connection is restarted (a PPPoE connection, whether from restarting the interface or the whole router) the state is not preserved and my IPv6 delegated prefix changes.
I don't know if my question is a feature request or a documentation request because the feature already exists.
Basically I'd like to be able to preserve the IA_PD and IA_NA between connection attempts/reboots etc. since unless my connection goes down for a prolonged period the prefix should not have been delegated to someone else yet and an explicit request for the prefix I already held should therefor work in most cases.
It could be that somehow allowing to set option 25 for the pppoe dhcp6 client would be sufficient, but it's not clear to me where I would do that /etc/config/dhcp is as far as I understand meant for local DHCP servers and not clients and /etc/config/network does not mention the ability to set this, also the DHCP client of the pppoe connection does not seem to be configured anywhere but just work magically.
It seems as if your ISP provides you with dynamic IPv4 and dynamic IPv6 prefixes, if that's the case, there is no retaining state on the DHCPv6 client - it's the ISP enforcing the IP/ prefix change.