Please define in this specific case what does “broken” means. I suspect there could be any other configuration errors here.
Promiscuous mode has to be enabled on the bridge interface on the Proxmox VE part, and it is not very well documented IIRC.
All you have to do is add the following line in the bridge interface stanza in /etc/network/interfaces
in Proxmox:
up /sbin/ip link set $IFACE promisc on
And reboot the host.