DHCPv6 relay always uses the "first" IPv6 address of an interface
Situation: you have multiple IPv6 addresses defined on an interface, such as a PD'd GUA and a locally defined ULA. You deal out the GUA via SLAAC, but want to use DHCPv6 for the ULAs and therefore enable the DHCPv6 relay
Problem: the relay will put the "first" address as relaying address, which can be the GUA, or like I have mentioned in the above issues, could be the ULA, it all depends. When it is the GUA however, the DHCPv6 server will refuse to serve the subnet since of course it does not know that prefix and has no IPs for it.