Looks like there is a bug in HP A5820 (or any Comwarev5 based equipment for that matter) in this subject.
This is how a Cisco device handles this:
http://www.cisco.com/c/en/us/products/collateral/ios-nx-os-software/enterprise-ipv6-solution/whitepaper_c11-689821.html
"
DHCPv6-PD Router: Installation of the route towards the CPE for the delegated prefix
Jan 28 02:18:55.620: IPv6RT0: static, Add 2001:DB8:1::/48 to table
Jan 28 02:18:55.620: IPv6RT0: static, Adding next-hop FE80::21A:2FFF:FE87:5602 over FastEthernet6/0 for 2001:DB8:1::/48, [1/0]
Jan 28 02:18:55.620: IPv6RT0: Event: 2001:DB8:1::/48, Add, owner static, previous None
! Note: If the Service Provider edge router made the binding between the CPE and its delegated prefix it will install automatically a static route for this delegated prefix into the routing table. The next-hop addresses used will be Link-Local.
"
I have filed a bug report towards HP about this, so lets keeps our fingers crossed that they will fix it...