Sometimes on DHCPv6 you need to request a specific PD, but unfortunately there is no such option
Add to settings prefix hint
set interfaces ethernet ethX dhcpv6-options pd 0 prefix fd12:3456:789a::/64
Configuration system
id-assoc pd 0 {
prefix ::/56 infinity;
prefix hint fd12:3456:789a::/64
prefix-interface ethX {
sla-len 8;
sla-id 0;
ifid 1;
};
};