I’ve reported a bug on bridge-utils, but perhaps someone has already seen this and has a fix. My virtual IPv6 machines often lose connectivity from time to time. Tracking this down, it seems that the router sends Neighbor Solicitations (IPv6 ARPs basically). The physical interface of the bridge group receives it, but the vnet0 one does not.
Using tshark I can see the pings on vnet0 but on br0 and eth1 I see the ping requests and the NS packets. So there is something odd going on with the bridge interface.
If I remove and add the vnet0 interface from the bridge group, the connectivity comes back.