On debian 9, the network configuration is handled by systemd by default. The settings of the old configuration file /etc/network/interfaces are now in /etc/systemd/network/50-default.network
Webmin does not handled this new systemd config. It stick on /etc/network/interfaces.
You can see it by going to Webmin > Networking > Network Configuration >> Network Interfaces , the tab "Activated at Boot" is always empty on fresh installed systemd / Debian.
And so even you add a failover IP (an alias IP) with the systemd-way, Webmin never see it.
For notice : To add an alias IP on systemd, you should add this Address block at the end of the /etc/systemd/network/50-default.network file :
( note that a Label with a colon ":" will not work, ref: https://github.com/systemd/systemd/issues/4057 )
Adding manually by hand a configuration in /etc/network/interfaces will work, but you need to add the whole configuration for one interface (not only the alias IP for example), so you need to duplicate and rewrite the configuration from /etc/systemd/network/50-default.network.
It seems it's /etc/init.d/networking script that mount interfaces presents in /etc/network/interfaces (because it use ifquery command).
/etc/network/interfaces will also be not supported in future releases, it is now a "legacy" configuration file ( https://www.debian.org/doc/manuals/debian-reference/ch05.en.html#_the_le... ).
So, in fact, actually both /etc/network/interface and /etc/systemd/network/* can set network interfaces, but the default is /etc/systemd/network/*. Can you handled both of them in webmin ?