In the situation, when we want to use more than one DHCPv6 "upper" interface in DHCPv6-Relay, dhcpv6_relay.py generates wrong OPTIONS. Multiple upstream_addr is joining with -u but must be joined with -u (with additional space at the start). In another case, dhcrelay can't start, because OPTIONS looks like:
OPTIONS="-6 -l XXXX:XXXX:XXXX%eth1 -u XXXX:XXXX:XXXX%eth2-u XXXX:XXXX:XXXX%eth3"
Also, we can't configure multiple upstream DHCPv6 servers in config on the same upstream-interface: instead of adding new IPv6 addresses to an upstream-interface configuration, they are replacing.