That is exactly the problem. But there is a solution. To circumvent connman to shutdown the interface on reboot/shutdown you need to prevent connman to manage the interface.
Connman has an option to ignore interfaces via a blacklist in its configuration. The configuration is hard coded in squashfs image. But there is a script (/usr/lib/connman/connman-setup) that checks for a custom config before starting connman. Simply copy /etc/connman/main.conf to /storage/.config/connman_main.conf and add the network interface to NetworkInterfaceBlacklist option.
That does work, but it causes other problems. With the ethernet device no longer managed by connmand, the DNS configuration can't be changed because it's hard coded into the systemd unit file for the connman service. DNS1 and DNS2 get set to Google's DNS servers.
I tried to set the DNS servers in the connman_main.conf file and it failed to take because /etc/resolv.conf is actually a symlink to a connman dns config file. I don't want LibreELEC to be sending DNS queries to Google's servers: I have my own internal DNS server that I'd rather use and in fact have to because my database is centralized on a MariaDB KVM. With LibreELEC set to use Google's DNS servers, Kodi can't find the library.
It also seems the default route isn't being set properly because LibreELEC can't reach the Internet (e.g. addon downloads fail).
Is there another workaround I can try? I've been thinking that maybe I can setup a cron job on a server to check if the LibreELEC systems are up and if they are, to login and change the systemd config so that shutdowns and reboots work properly. The only problem with my craptastic idea is that I don't know for sure what's happening: is the file system being unmounted too soon or is the network shutting down too early?