tested on precise:
using isc-dhcp-client 4.1.ESV-R4-0ubuntu5.10, bug exists; dhcpv6 prefix is set to /64.
using isc-dhcp-client 4.1.ESV-R4-0ubuntu5.11, bug is fixed; dhcpv6 prefix is set correctly to /128.
tested on precise:
using isc-dhcp-client 4.1.ESV- R4-0ubuntu5. 10, bug exists; dhcpv6 prefix is set to /64.
using isc-dhcp-client 4.1.ESV- R4-0ubuntu5. 11, bug is fixed; dhcpv6 prefix is set correctly to /128.