Adrian,
The /etc/network/interfaces file settings will get overridden by the API process on boot. The information in the interfaces file will get applied early in the boot process, but when the API starts it will override those settings with what is in the /var/config/db.json.
You can use the “if” commands or ifconfig to bring the interface down and then back up to apply the interfaces settings. If you want the device to apply the settings in the /etc/network/interfaces file you should log into the Web UI once you have access and update the eth0 settings to what you want them to be.
Jeff