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; }; };