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)