HA with prefix delegation: Wrong gateway announced

[ajr]

In this HA setup, there are some VLANS, which are associated to WLANS by
unifi accesspoints. Each of them has a different subnet (by prefix id).
Prefix delegation works correctly and assigns addresses from the right prefix
to the mobile clients (using Track interface).
The setup is the same as described here https://forum.opnsense.org/index.php?topic=50602.msg259211#msg259211
The IP6 gateway announced in the past was that from system->gateways
(WAN_DHCP6), which is the LLA of the DSL-Router (fe80::cece:1eff:feb3:757f) and worked perfectly on failover.
This changed recently to LLAs like fe80::20d:b9ff:fe48:5c1e, which are unusable.

How can I force radvd to announce WAN_DHCP6 ?

Version:  26.7.8_2