I had update proxmox to latest version just now. It occurs problem. When I reboot LXC, LXC's mac address will change automatic. It makes me impossible to bind static address.
there was a bug[0] where adding a network interface to a container did not generate a mac address in the config,
check if there is a 'hwaddr' property on the network in the config (/etc/pve/nodes/NODENAME/lxc/ID.conf)
if not, add one to get a static mac (or use the webinterface for it)