(first post here, so please yell at me if I do anything wrong)
Unfortunately the various suggestions such as setting up a mount in /etc/fstab were not working for me, as I'm trying to develop software inside this VM, and NFS doesn't play well with various build tools, due to no support for kernel functionality called "inotify" (Facebook's watchman package depends on it).
I haven't 100% confirmed any of this, but I think I know what's happening? I at least have a basic understanding of my case?
It looks like a brand new Ubuntu 18.04 installation is no longer using /etc/network/interfaces for network config, and is instead using /etc/netplan/50-cloud-init.yaml (I don't know if it's always the same filename).
I assume that this is related to the cloud-init stuff that was added to Ubuntu in the recent past, based on the name of the file.
For some reason, this is configured correctly before mounting a share. After mounting a share, I found that /etc/netplan/50-cloud-init.yaml had the wrong network interface identifier.
I don't know if my network interface identifier changed, or if the contents of this file changed.
My network interface is now called enp3s0 so I edited the contents of the file to look like this:
network:
ethernets:
enp3s0:
dhcp4: true
version: 2
(read more about netplan config at https://netplan.io/examples)
And the problem is solved. I believe that the other suggestions in this thread are workarounds, and this technically could be too, since I haven't confirmed which bit changes after mounting the share, or why it happens.
If I find more, I will post back!
(side note: If Unraid supported EZ VM snapshots, it would be a lot easier to figure this out. As-is, I have to reinstall Ubuntu over and over...)
