I've found a small issue with the changes implemented for T3379

config.boot:

service {
    dhcpv6-server {
        global-parameters {
            name-server XXXX:XXXX:XXXX::8D
            name-server XXXX:XXXX:XXXX::8E
        }
    }
}

/run/dhcp-server/dhcpdv6.conf:

option dhcp6.name-servers XXXX:XXXX:XXXX::8D;
option dhcp6.name-servers XXXX:XXXX:XXXX::8E;

This configuration format is wrong.

dhcp6.name-servers is a comma-delimited, multi-value list of name-servers that should only appear once in the dhcpdv6.conf file.
If multiple dhcp6.name-servers entries are present, only the last entry is used.
The way vyos currently generates the dhcpdv6.conf file causes DHCPv6 clients to only receive information about a single name server.

Proper parsing of the vyos config would result in a dhcpdv6.conf that includes a single line like this:

option dhcp6.name-servers XXXX:XXXX:XXXX::8D,XXXX:XXXX:XXXX::8E;