Preface: I'm very new to ipv6. Assume i know nothing.
For management reasons, i'd like to run a dual stack LAN network, where I have full control over the IPv6 suffixes that are being handed out (i.e. DHCPv6, operating as DHCPv4 normally does).
To achieve this, i've set option ra_management '2', works great.
The unexpected (at least to me) side effect of this, is that the WAN (eth0.2) interface no longer receives an IPv6 address for itself (i.e. xxx/128).
Is there a way of operating in stateful mode, where WAN still receives a /128 via dhcpv6?
That is strange and curious indeed, maybe a bug in netifd (network manager).
Did you try snapshots from the 19.07 a/o Master branches, assuming the node is currently on 18.06.x? Some codes developments are not riding the trains (backported) and thus snapshots from more contemporary branches may sort the matter.
Noticed that WAN has DHCP set for both ipv4 and ipv6 - is the router connected directly to the ISP subscriber line or being downstream of some (ISP provided) device (modem/router) and/or switch device?
Well, that says what you want to give both DHCP and slaac addresses (ra) on LAN.
If you want only dhcpv6 in LAN, remove ra or set it disabled. That might achieve what you want, even with ra_management 1.
Aren't you mixing things? Your ISP controls the upstream IPv6 address given to your router in wan.