Disappearing Bridge

[UhostWguest]

I'm running VirtualBox 6.0 on an Ubuntu 18.04.1 host, with a Windows 10 guest configured to use a host only network with fixed IP address, though I've also tried using DHCP.

In my Ubuntu host, I've configured a bridge using network manager (nmcli) to bridge four physical network interfaces to a fixed IP, all the IP addresses in the whole system are in the same class C network space, and this network space is "for internal use only" - there's another physical ethernet port that will be used to talk to outside networks, and it is typically configured to use DHCP.

When we were using 192.168.123.x as our internal network number space, everything was working fine. We'd like to move out of 192.168.x.x to avoid potential conflicts as many hundreds of copies of this device find homes on various networks around the world, networks we do not control. Since the internal network never talks to the outside world, we thought a test net like 198.51.100.x might be a good way for us to avoid conflicts between our internal address space and whatever might be assigned from unknown DHCP servers.

The problem we are having is: when VirtualBox starts up with the host only network configured in the test net IP address space, it immediately knocks off all our functionality in the nmcli created bridge. Before vboxnet0 is created, everything is communicating as expected, but as soon as vboxnet0 is created, we can no longer communicate with any devices on "ourbridge1". The software in our Windows guest doesn't need to communicate with the devices on the bridge/physical ports, but it does need to communicate with a server running in the Ubuntu host - which it can do, but nothing in the host can reach devices on ourbridge1.

Does this "disappearing bridge" sound familiar to anyone? Any suggestions (beyond resigning and moving all IP addresses back to the 192.168 space) for how to keep ourbridge1 from becoming disabled?