I have been using Webmin for many years on Debian systems I have installed manually, on these I’ve always put the network config in the /etc/network/interfaces file.
I’ve just created a VPS at a hosting company and their default config installs network info in /etc/network/interfaces.d/50-clout-init
Webmin fails to read/write to this config file, so only shows current config and it’s not possible to manage boot time config.
Webmin needs to follow improvements in Debian and allow modern config methods to be handled.
This is not a modern config method. The /etc/network/interfaces scripts are being deprecated and have been in the process of being retired for several years, being replaced by either systemd-networkd, netplan (maybe only on Ubuntu? edit: nope, also available in Debian: netplan(5) — netplan.io — Debian bullseye — Debian Manpages), or NetworkManager.
That said, I thought Webmin did read files in /etc/network/interfaces.d. I’m not sure why you’re not seeing them. Can you post your actual /etc/network/interfaces and that file in /etc/network/interfaces.d?