I just encountered this same issue. In addition to the 'ugly workaround', I
also needed to explicitly add the default route otherwise my system did not
have any outbound IPv6 connectivity:
auto br0
iface br0 inet dhcp
bridge_ports eth0
bridge_fd 0
bridge_maxwait 0
# https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=319832
up ip -6 addr add 2605:6400:20:92e::1337:c0de/64 dev br0
up ip -6 route add 2605:6400:20::1/128 dev br0
up ip -6 route add default via 2605:6400:20::1
down ip -6 addr del 2605:6400:20:92e::1337:c0de/64 dev br0
down ip -6 route del default via 2605:6400:20::1
down ip -6 route del 2605:6400:20::1/128 dev br0
(in my this, 2605:6400:20:92e::1337:c0de is the server's IP while
2605:6400:20::1 is the gateway IP)
