For anyone coming later, Ubuntu server use's netplan for configuration via files in /etc/netplan and you should not configure networking directly via systemd-networkd .network files.
More details here.
In my case if I ran sudo cat /etc/netplan/00-installer-config.yaml I got the following:
# This is the network config written by 'subiquity'
network:
ethernets:
enp2s0:
dhcp4: true
optional: true
version: 2
The specific issue was that I had optional: true which I had done when I first setup this system as I originally had it setup to use wireless while doing the initial configuration and then switched to hard wired once I deployed it.
Changing optional: true to optional: false, then removing the .network file I had put in /etc/systemd/network, and then rebooting I can now see that cat /run/systemd/network/10-netplan-enp2s0.network no longer has RequiredForOnline=no.
Apparently the default is to require the network adapter to be in a routable state so now with the correct netplan configuration I get the right generated systemd-networkd configuration which properly waits for the interface to be routable before trying to mount volumes and all volumes mount as expected without intervention.
The root cause behind this issue was me making a configuration change I meant to be temporary but then forgot to reverse and then because I didn't understand netplan my troubleshooting was in the wrong places.
