I created a new bridge but it's not up after boot

[jimmyjoe]

I created a bridge (vmbr3) yesterday and rebooted my proxmox hypervisor but vmbr3 is not coming up at boot and the VM using this bridge is failing to start automatically. I can `ifup vmbr3` and start the VM by hand, but I'd like to know what's not working correctly.

My /etc/network/interfaces is here

auto vmbr3
iface vmbr3 inet static
   address  10.10.10.27
   netmask  255.255.255.0
   bridge_ports eth3
   bridge_stp off
   bridge_fd 0

I also noticed this hidden file in /etc/network/. Could this be the issue?

# dir /etc/network
...
-rw-r--r--   1 root root    0 Apr 17  2017 .pve-interfaces.lock
...